<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019cd357-7159-705c-b76d-c4f430d9b33c,g:ad3398f8-776b-4b62-b107-5796aded0cca-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/CondensedConsolidatedBalanceSheets" xlink:type="simple" xlink:href="mnk-20260331.xsd#CondensedConsolidatedBalanceSheets"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/CondensedConsolidatedBalanceSheets" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019cd357-7327-72e2-837d-9e0c8df317c3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019cd357-7327-7bc9-bbaa-63c8c96e8c7d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019cd357-7327-72e2-837d-9e0c8df317c3" xlink:to="loc_us-gaap_StatementTable_019cd357-7327-7bc9-bbaa-63c8c96e8c7d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019cd357-7327-7be9-a3b6-691cd072c61a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019cd357-7327-7bc9-bbaa-63c8c96e8c7d" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019cd357-7327-7be9-a3b6-691cd072c61a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019cd357-7327-7be9-a3b6-691cd072c61a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019cd357-7327-7be9-a3b6-691cd072c61a" xlink:to="loc_us-gaap_ClassOfStockDomain_019cd357-7327-7be9-a3b6-691cd072c61a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019cd357-7327-7967-a00c-46c4d713ead5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019cd357-7327-7be9-a3b6-691cd072c61a" xlink:to="loc_us-gaap_ClassOfStockDomain_019cd357-7327-7967-a00c-46c4d713ead5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019cd357-7327-72bc-9cc2-46a71df25af5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019cd357-7327-7967-a00c-46c4d713ead5" xlink:to="loc_us-gaap_CommonClassAMember_019cd357-7327-72bc-9cc2-46a71df25af5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019cd357-7327-738e-bf47-397fde299ad6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019cd357-7327-7967-a00c-46c4d713ead5" xlink:to="loc_us-gaap_CommonStockMember_019cd357-7327-738e-bf47-397fde299ad6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_019cd357-7327-7d7b-833f-217f05610270" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019cd357-7327-72e2-837d-9e0c8df317c3" xlink:to="loc_us-gaap_AssetsAbstract_019cd357-7327-7d7b-833f-217f05610270" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrentAbstract_019cd357-7327-7953-b9dd-4528e3b0be7a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019cd357-7327-7d7b-833f-217f05610270" xlink:to="loc_us-gaap_AssetsCurrentAbstract_019cd357-7327-7953-b9dd-4528e3b0be7a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019cd357-7327-7bd8-aa94-8ea9c921a90b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019cd357-7327-7953-b9dd-4528e3b0be7a" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_019cd357-7327-7bd8-aa94-8ea9c921a90b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_019cd357-7327-7014-b705-da5dc6608259" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsReceivableNetCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019cd357-7327-7953-b9dd-4528e3b0be7a" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_019cd357-7327-7014-b705-da5dc6608259" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryNet_019cd357-7327-7dd5-8617-64a51ba89d23" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019cd357-7327-7953-b9dd-4528e3b0be7a" xlink:to="loc_us-gaap_InventoryNet_019cd357-7327-7dd5-8617-64a51ba89d23" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_019cd357-7327-7210-aa2d-b42ff44614c7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019cd357-7327-7953-b9dd-4528e3b0be7a" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_019cd357-7327-7210-aa2d-b42ff44614c7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsCurrent_019cd357-7327-7ed0-9b38-577e65953bb4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_019cd357-7327-7953-b9dd-4528e3b0be7a" xlink:to="loc_us-gaap_AssetsCurrent_019cd357-7327-7ed0-9b38-577e65953bb4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_019cd357-7327-7348-9431-513a03550224" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019cd357-7327-7d7b-833f-217f05610270" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_019cd357-7327-7348-9431-513a03550224" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019cd357-7327-7932-873b-4fb92b619c28" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019cd357-7327-7d7b-833f-217f05610270" xlink:to="loc_us-gaap_Goodwill_019cd357-7327-7932-873b-4fb92b619c28" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_019cd357-7327-7d9c-aae4-602c3b9ae13e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019cd357-7327-7d7b-833f-217f05610270" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_019cd357-7327-7d9c-aae4-602c3b9ae13e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxAssetsNet_019cd357-7327-737b-b5a2-5448a9d4d5f6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019cd357-7327-7d7b-833f-217f05610270" xlink:to="loc_us-gaap_DeferredIncomeTaxAssetsNet_019cd357-7327-737b-b5a2-5448a9d4d5f6" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_019cd357-7327-7346-8eb8-514e65be0440" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019cd357-7327-7d7b-833f-217f05610270" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_019cd357-7327-7346-8eb8-514e65be0440" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Assets_019cd357-7327-7743-962d-0704d42658fa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Assets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_019cd357-7327-7d7b-833f-217f05610270" xlink:to="loc_us-gaap_Assets_019cd357-7327-7743-962d-0704d42658fa" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019cd357-7327-7578-9a54-436d1710fcab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019cd357-7327-72e2-837d-9e0c8df317c3" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019cd357-7327-7578-9a54-436d1710fcab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_019cd357-7327-78f3-8052-0cd7890dbb7e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019cd357-7327-7578-9a54-436d1710fcab" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_019cd357-7327-78f3-8052-0cd7890dbb7e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_019cd357-7327-7d8f-957e-de5c58fa9c1f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019cd357-7327-78f3-8052-0cd7890dbb7e" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligationsCurrent_019cd357-7327-7d8f-957e-de5c58fa9c1f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsPayableCurrent_019cd357-7327-7550-b2c4-684dbc29d449" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019cd357-7327-78f3-8052-0cd7890dbb7e" xlink:to="loc_us-gaap_AccountsPayableCurrent_019cd357-7327-7550-b2c4-684dbc29d449" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_019cd357-7327-7a42-9532-f4ad5930b276" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeRelatedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019cd357-7327-78f3-8052-0cd7890dbb7e" xlink:to="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_019cd357-7327-7a42-9532-f4ad5930b276" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPayableCurrent_019cd357-7327-7a32-9355-dfe01a85f90b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestPayableCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019cd357-7327-78f3-8052-0cd7890dbb7e" xlink:to="loc_us-gaap_InterestPayableCurrent_019cd357-7327-7a32-9355-dfe01a85f90b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_ActharGelRelatedSettlementCurrent_019cd357-7327-79bc-87ee-5b27d15d14a0" xlink:href="mnk-20260331.xsd#mnk_ActharGelRelatedSettlementCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019cd357-7327-78f3-8052-0cd7890dbb7e" xlink:to="loc_mnk_ActharGelRelatedSettlementCurrent_019cd357-7327-79bc-87ee-5b27d15d14a0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_AccruedRebatesAndReturnsCurrent_019cd357-7327-7a7e-8b00-9b8a65de1b25" xlink:href="mnk-20260331.xsd#mnk_AccruedRebatesAndReturnsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019cd357-7327-78f3-8052-0cd7890dbb7e" xlink:to="loc_mnk_AccruedRebatesAndReturnsCurrent_019cd357-7327-7a7e-8b00-9b8a65de1b25" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesCurrent_019cd357-7327-7f07-8970-8fb5d795cb8e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019cd357-7327-78f3-8052-0cd7890dbb7e" xlink:to="loc_us-gaap_OtherLiabilitiesCurrent_019cd357-7327-7f07-8970-8fb5d795cb8e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrent_019cd357-7327-7bbf-9755-3eb219d19a76" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_019cd357-7327-78f3-8052-0cd7890dbb7e" xlink:to="loc_us-gaap_LiabilitiesCurrent_019cd357-7327-7bbf-9755-3eb219d19a76" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_019cd357-7327-7b63-b997-0997452b0c88" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtAndCapitalLeaseObligations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019cd357-7327-7578-9a54-436d1710fcab" xlink:to="loc_us-gaap_LongTermDebtAndCapitalLeaseObligations_019cd357-7327-7b63-b997-0997452b0c88" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_ActharGelRelatedSettlementNonCurrent_019cd357-7327-73c4-aeeb-efa6977afc59" xlink:href="mnk-20260331.xsd#mnk_ActharGelRelatedSettlementNonCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019cd357-7327-7578-9a54-436d1710fcab" xlink:to="loc_mnk_ActharGelRelatedSettlementNonCurrent_019cd357-7327-73c4-aeeb-efa6977afc59" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_019dbeb9-f819-70f4-a8e5-a9624e9f4e38" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredIncomeTaxLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019cd357-7327-7578-9a54-436d1710fcab" xlink:to="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_019dbeb9-f819-70f4-a8e5-a9624e9f4e38" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedIncomeTaxesNoncurrent_019cd357-7327-75ac-91e5-120594624bea" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccruedIncomeTaxesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019cd357-7327-7578-9a54-436d1710fcab" xlink:to="loc_us-gaap_AccruedIncomeTaxesNoncurrent_019cd357-7327-75ac-91e5-120594624bea" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_019cd357-7327-72fe-bc73-140488f60003" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherLiabilitiesNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019cd357-7327-7578-9a54-436d1710fcab" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_019cd357-7327-72fe-bc73-140488f60003" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Liabilities_019cd357-7327-75b0-a140-6adcab963b50" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Liabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019cd357-7327-7578-9a54-436d1710fcab" xlink:to="loc_us-gaap_Liabilities_019cd357-7327-75b0-a140-6adcab963b50" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommitmentsAndContingencies_019cd357-7327-75ed-beb5-5cc1a4d56185" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommitmentsAndContingencies"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019cd357-7327-7578-9a54-436d1710fcab" xlink:to="loc_us-gaap_CommitmentsAndContingencies_019cd357-7327-75ed-beb5-5cc1a4d56185" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityAbstract_019cd357-7327-7548-8fa4-8572bcfbb0fe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019cd357-7327-7578-9a54-436d1710fcab" xlink:to="loc_us-gaap_StockholdersEquityAbstract_019cd357-7327-7548-8fa4-8572bcfbb0fe" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockValue_019cd357-7327-7c71-9dd6-ea3781787fce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019cd357-7327-7548-8fa4-8572bcfbb0fe" xlink:to="loc_us-gaap_PreferredStockValue_019cd357-7327-7c71-9dd6-ea3781787fce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockValueOutstanding_019cd357-7327-7d56-9c7d-04a5dbbaf14c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockValueOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019cd357-7327-7548-8fa4-8572bcfbb0fe" xlink:to="loc_us-gaap_CommonStockValueOutstanding_019cd357-7327-7d56-9c7d-04a5dbbaf14c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapital_019cd357-7327-7dc5-9c11-c2910e42497f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapital"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019cd357-7327-7548-8fa4-8572bcfbb0fe" xlink:to="loc_us-gaap_AdditionalPaidInCapital_019cd357-7327-7dc5-9c11-c2910e42497f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019cd357-7327-7a4f-bd56-3539079c44fd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019cd357-7327-7548-8fa4-8572bcfbb0fe" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_019cd357-7327-7a4f-bd56-3539079c44fd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_019cd357-7327-7cd9-9e2d-301d79444ca0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsAccumulatedDeficit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019cd357-7327-7548-8fa4-8572bcfbb0fe" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_019cd357-7327-7cd9-9e2d-301d79444ca0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019cd357-7327-70b0-a48b-81075ced21eb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_019cd357-7327-7548-8fa4-8572bcfbb0fe" xlink:to="loc_us-gaap_StockholdersEquity_019cd357-7327-70b0-a48b-81075ced21eb" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_019cd357-7327-7ed3-bce1-870c8bda474a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesAndStockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_019cd357-7327-7578-9a54-436d1710fcab" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_019cd357-7327-7ed3-bce1-870c8bda474a" xlink:type="arc" order="10"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/CondensedConsolidatedBalanceSheetsParenthetical" xlink:type="simple" xlink:href="mnk-20260331.xsd#CondensedConsolidatedBalanceSheetsParenthetical"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/CondensedConsolidatedBalanceSheetsParenthetical" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019cd357-7327-7461-9040-74715795fc91" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019cd357-7327-7abe-aafd-1b0ebcada5b5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019cd357-7327-7461-9040-74715795fc91" xlink:to="loc_us-gaap_StatementTable_019cd357-7327-7abe-aafd-1b0ebcada5b5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_019cd357-7327-7007-a438-2c7eb9919e1e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019cd357-7327-7abe-aafd-1b0ebcada5b5" xlink:to="loc_us-gaap_StatementClassOfStockAxis_019cd357-7327-7007-a438-2c7eb9919e1e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019cd357-7327-7007-a438-2c7eb9919e1e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019cd357-7327-7007-a438-2c7eb9919e1e" xlink:to="loc_us-gaap_ClassOfStockDomain_019cd357-7327-7007-a438-2c7eb9919e1e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_019cd357-7327-7a38-8bc7-9f9c5cf846a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_019cd357-7327-7007-a438-2c7eb9919e1e" xlink:to="loc_us-gaap_ClassOfStockDomain_019cd357-7327-7a38-8bc7-9f9c5cf846a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonClassAMember_019cd357-7327-7dcb-85e6-4b85c36f024c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonClassAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019cd357-7327-7a38-8bc7-9f9c5cf846a0" xlink:to="loc_us-gaap_CommonClassAMember_019cd357-7327-7dcb-85e6-4b85c36f024c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019cd357-7327-7700-a678-e46e5b93f516" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_019cd357-7327-7a38-8bc7-9f9c5cf846a0" xlink:to="loc_us-gaap_CommonStockMember_019cd357-7327-7700-a678-e46e5b93f516" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllowanceForDoubtfulAccountsReceivable_019cd357-7327-79b9-945c-5ec5b394cb54" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllowanceForDoubtfulAccountsReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019cd357-7327-7461-9040-74715795fc91" xlink:to="loc_us-gaap_AllowanceForDoubtfulAccountsReceivable_019cd357-7327-79b9-945c-5ec5b394cb54" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockParOrStatedValuePerShare_019cd357-7327-7689-b5ec-6b1ca8623e96" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019cd357-7327-7461-9040-74715795fc91" xlink:to="loc_us-gaap_PreferredStockParOrStatedValuePerShare_019cd357-7327-7689-b5ec-6b1ca8623e96" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesAuthorized_019cd357-7327-789c-bb6a-e51c50dd7648" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019cd357-7327-7461-9040-74715795fc91" xlink:to="loc_us-gaap_PreferredStockSharesAuthorized_019cd357-7327-789c-bb6a-e51c50dd7648" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesIssued_019cd357-7327-7af2-b94e-24072b2e4890" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019cd357-7327-7461-9040-74715795fc91" xlink:to="loc_us-gaap_PreferredStockSharesIssued_019cd357-7327-7af2-b94e-24072b2e4890" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PreferredStockSharesOutstanding_019cd357-7327-7679-8210-e935859a6d88" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PreferredStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019cd357-7327-7461-9040-74715795fc91" xlink:to="loc_us-gaap_PreferredStockSharesOutstanding_019cd357-7327-7679-8210-e935859a6d88" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_019cd357-7327-70f7-ace8-243e8e276124" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockParOrStatedValuePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019cd357-7327-7461-9040-74715795fc91" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_019cd357-7327-70f7-ace8-243e8e276124" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_019cd357-7327-7764-9626-425070fbe795" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019cd357-7327-7461-9040-74715795fc91" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_019cd357-7327-7764-9626-425070fbe795" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesIssued_019cd357-7327-74cd-a40a-9a5a3e82cfe7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019cd357-7327-7461-9040-74715795fc91" xlink:to="loc_us-gaap_CommonStockSharesIssued_019cd357-7327-74cd-a40a-9a5a3e82cfe7" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_019cd357-7327-7b67-a75f-ca2648227b0c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019cd357-7327-7461-9040-74715795fc91" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_019cd357-7327-7b67-a75f-ca2648227b0c" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/CondensedConsolidatedStatementsofChangesinShareholdersEquity" xlink:type="simple" xlink:href="mnk-20260331.xsd#CondensedConsolidatedStatementsofChangesinShareholdersEquity"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/CondensedConsolidatedStatementsofChangesinShareholdersEquity" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019cd357-73c2-7379-91ac-4d58bce37e7a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019cd357-73c2-744b-a2b6-e5fdb5c2db21" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019cd357-73c2-7379-91ac-4d58bce37e7a" xlink:to="loc_us-gaap_StatementTable_019cd357-73c2-744b-a2b6-e5fdb5c2db21" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019cd357-73c2-7205-9497-98b04af050d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019cd357-73c2-744b-a2b6-e5fdb5c2db21" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019cd357-73c2-7205-9497-98b04af050d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019cd357-73c2-7205-9497-98b04af050d4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019cd357-73c2-7205-9497-98b04af050d4" xlink:to="loc_us-gaap_EquityComponentDomain_019cd357-73c2-7205-9497-98b04af050d4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019cd357-73c2-786a-914c-bda409f142de" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019cd357-73c2-7205-9497-98b04af050d4" xlink:to="loc_us-gaap_EquityComponentDomain_019cd357-73c2-786a-914c-bda409f142de" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019cd357-73c2-7cd3-b29b-d3a20afb9077" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019cd357-73c2-786a-914c-bda409f142de" xlink:to="loc_us-gaap_CommonStockMember_019cd357-73c2-7cd3-b29b-d3a20afb9077" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonMember_019cd357-73c2-7e76-b7d9-6dc4a39d9526" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockCommonMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019cd357-73c2-786a-914c-bda409f142de" xlink:to="loc_us-gaap_TreasuryStockCommonMember_019cd357-73c2-7e76-b7d9-6dc4a39d9526" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019cd357-73c2-7c95-a36a-cfa9a38662b0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019cd357-73c2-786a-914c-bda409f142de" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019cd357-73c2-7c95-a36a-cfa9a38662b0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019cd357-73c2-76f8-8633-2e8f92b75a08" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019cd357-73c2-786a-914c-bda409f142de" xlink:to="loc_us-gaap_RetainedEarningsMember_019cd357-73c2-76f8-8633-2e8f92b75a08" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019cd357-73c2-77ed-954d-9202a374f49d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019cd357-73c2-786a-914c-bda409f142de" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019cd357-73c2-77ed-954d-9202a374f49d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cd357-73c2-74b6-b21f-ada3d48a9577" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019cd357-73c2-7379-91ac-4d58bce37e7a" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cd357-73c2-74b6-b21f-ada3d48a9577" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_019cd357-73c2-713d-8800-8fa6e2bd4360" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cd357-73c2-74b6-b21f-ada3d48a9577" xlink:to="loc_us-gaap_SharesOutstanding_019cd357-73c2-713d-8800-8fa6e2bd4360" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019cd357-73c2-7158-bc0d-bf05c319f2ad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cd357-73c2-74b6-b21f-ada3d48a9577" xlink:to="loc_us-gaap_StockholdersEquity_019cd357-73c2-7158-bc0d-bf05c319f2ad" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_019cd357-73c2-7364-8340-799c299250e8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockCommonShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cd357-73c2-74b6-b21f-ada3d48a9577" xlink:to="loc_us-gaap_TreasuryStockCommonShares_019cd357-73c2-7364-8340-799c299250e8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019cd357-73c2-7011-830b-d86ad1405e8b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cd357-73c2-74b6-b21f-ada3d48a9577" xlink:to="loc_us-gaap_NetIncomeLoss_019cd357-73c2-7011-830b-d86ad1405e8b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_019d9b38-dc31-7c78-a5ae-9483b03d74e9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cd357-73c2-74b6-b21f-ada3d48a9577" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTax_019d9b38-dc31-7c78-a5ae-9483b03d74e9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019cd357-73c2-7d2c-b1e1-333c2b69adff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cd357-73c2-74b6-b21f-ada3d48a9577" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesShareBasedCompensation_019cd357-73c2-7d2c-b1e1-333c2b69adff" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockValueSharesIssuedNetOfTaxWithholdings_019cd357-73c2-742a-828c-e54fa35068d8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockValueSharesIssuedNetOfTaxWithholdings"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cd357-73c2-74b6-b21f-ada3d48a9577" xlink:to="loc_us-gaap_RestrictedStockValueSharesIssuedNetOfTaxWithholdings_019cd357-73c2-742a-828c-e54fa35068d8" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_CommonStockAmountCancelled_019cd357-73c2-73fb-9d20-41ca032ef8e1" xlink:href="mnk-20260331.xsd#mnk_CommonStockAmountCancelled"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cd357-73c2-74b6-b21f-ada3d48a9577" xlink:to="loc_mnk_CommonStockAmountCancelled_019cd357-73c2-73fb-9d20-41ca032ef8e1" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019cd357-73c2-71db-8a73-ea88e7192d07" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019cd357-73c2-74b6-b21f-ada3d48a9577" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019cd357-73c2-71db-8a73-ea88e7192d07" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesOutstanding_019cd357-73c2-7418-bed0-fe9ad57ae6d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharesOutstanding"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquity_019cd357-73c2-70ec-b4b6-408e0125ab92" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquity"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockCommonShares_019cd357-73c2-73c9-a0e5-6dc2f47ed609" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockCommonShares"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/BusinessCombinationDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#BusinessCombinationDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/BusinessCombinationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_019e021c-19bf-79ec-99eb-3d4d37227505" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019e021c-19bf-721c-894d-85ca8e4343c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e021c-19bf-79ec-99eb-3d4d37227505" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019e021c-19bf-721c-894d-85ca8e4343c4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019e021c-19bf-733c-8fe7-cba6658d0f52" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019e021c-19bf-721c-894d-85ca8e4343c4" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019e021c-19bf-733c-8fe7-cba6658d0f52" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e021c-19bf-733c-8fe7-cba6658d0f52_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019e021c-19bf-733c-8fe7-cba6658d0f52" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e021c-19bf-733c-8fe7-cba6658d0f52_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e021c-19bf-790a-bd16-3647ba7ce872" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019e021c-19bf-733c-8fe7-cba6658d0f52" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e021c-19bf-790a-bd16-3647ba7ce872" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_MallinckrodtEndoMergerMember_019e021c-19bf-7415-88a3-5f937c194a4a" xlink:href="mnk-20260331.xsd#mnk_MallinckrodtEndoMergerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019e021c-19bf-790a-bd16-3647ba7ce872" xlink:to="loc_mnk_MallinckrodtEndoMergerMember_019e021c-19bf-7415-88a3-5f937c194a4a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentInventory_019e021c-19bf-7782-a46c-b824c35208f6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentInventory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e021c-19bf-79ec-99eb-3d4d37227505" xlink:to="loc_us-gaap_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentInventory_019e021c-19bf-7782-a46c-b824c35208f6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentPrepaidExpenseAndOtherAssetsCurrent_019e021c-19bf-7ad4-bd8b-3d09467de6a6" xlink:href="mnk-20260331.xsd#mnk_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentPrepaidExpenseAndOtherAssetsCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e021c-19bf-79ec-99eb-3d4d37227505" xlink:to="loc_mnk_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentPrepaidExpenseAndOtherAssetsCurrent_019e021c-19bf-7ad4-bd8b-3d09467de6a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherLiabilityCurrent_019e021c-19bf-76b2-8c7b-83311a2c99bb" xlink:href="mnk-20260331.xsd#mnk_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e021c-19bf-79ec-99eb-3d4d37227505" xlink:to="loc_mnk_BusinessCombinationProvisionalInformationInitialAccountingIncompleteAdjustmentOtherLiabilityCurrent_019e021c-19bf-76b2-8c7b-83311a2c99bb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillPurchaseAccountingAdjustments_019e021e-9707-763b-9bf6-473361a4d786" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GoodwillPurchaseAccountingAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019e021c-19bf-79ec-99eb-3d4d37227505" xlink:to="loc_us-gaap_GoodwillPurchaseAccountingAdjustments_019e021e-9707-763b-9bf6-473361a4d786" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/DivestituresIncomeLossFromDiscontinuedOperationsDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#DivestituresIncomeLossFromDiscontinuedOperationsDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/DivestituresIncomeLossFromDiscontinuedOperationsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-9e2e-76b7-be06-331d6ba4d415" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_019dab70-9e2e-7025-9fed-78d119a89624" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-9e2e-76b7-be06-331d6ba4d415" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_019dab70-9e2e-7025-9fed-78d119a89624" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019dab70-9e2e-7e6d-9f76-516645933211" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_019dab70-9e2e-7025-9fed-78d119a89624" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019dab70-9e2e-7e6d-9f76-516645933211" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019dab70-9e2e-7e6d-9f76-516645933211_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019dab70-9e2e-7e6d-9f76-516645933211" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019dab70-9e2e-7e6d-9f76-516645933211_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019dab70-9e2e-74b5-a807-35a8777dd253" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019dab70-9e2e-7e6d-9f76-516645933211" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019dab70-9e2e-74b5-a807-35a8777dd253" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_ParHealthMember_019dab70-9e2e-730c-aa1e-e98332a79586" xlink:href="mnk-20260331.xsd#mnk_ParHealthMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019dab70-9e2e-74b5-a807-35a8777dd253" xlink:to="loc_mnk_ParHealthMember_019dab70-9e2e-730c-aa1e-e98332a79586" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_NuclearImagingMember_019dbed9-3deb-7657-aba2-2d7f6d245a01" xlink:href="mnk-20260331.xsd#mnk_NuclearImagingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019dab70-9e2e-74b5-a807-35a8777dd253" xlink:to="loc_mnk_NuclearImagingMember_019dbed9-3deb-7657-aba2-2d7f6d245a01" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationAxis_019dab70-9e2e-79ed-8b0d-ee452a980ac9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_019dab70-9e2e-7025-9fed-78d119a89624" xlink:to="loc_us-gaap_DisposalGroupClassificationAxis_019dab70-9e2e-79ed-8b0d-ee452a980ac9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_019dab70-9e2e-79ed-8b0d-ee452a980ac9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_019dab70-9e2e-79ed-8b0d-ee452a980ac9" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_019dab70-9e2e-79ed-8b0d-ee452a980ac9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_019dab70-9e2e-7497-b587-94b9a47bb5ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_019dab70-9e2e-79ed-8b0d-ee452a980ac9" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_019dab70-9e2e-7497-b587-94b9a47bb5ee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationsDisposedOfByMeansOtherThanSaleSpinoffMember_019dab70-9e2e-731e-a505-349ea941a321" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DiscontinuedOperationsDisposedOfByMeansOtherThanSaleSpinoffMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupClassificationDomain_019dab70-9e2e-7497-b587-94b9a47bb5ee" xlink:to="loc_us-gaap_DiscontinuedOperationsDisposedOfByMeansOtherThanSaleSpinoffMember_019dab70-9e2e-731e-a505-349ea941a321" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationsDisposedOfBySaleMember_019dbed9-3deb-7dfb-a010-822c5f2f967e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DiscontinuedOperationsDisposedOfBySaleMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupClassificationDomain_019dab70-9e2e-7497-b587-94b9a47bb5ee" xlink:to="loc_us-gaap_DiscontinuedOperationsDisposedOfBySaleMember_019dbed9-3deb-7dfb-a010-822c5f2f967e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue_019dab70-9e2e-728e-b5eb-11dcbd7d6368" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-9e2e-76b7-be06-331d6ba4d415" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationRevenue_019dab70-9e2e-728e-b5eb-11dcbd7d6368" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationCostsOfGoodsSold_019dab70-9e2e-70d3-94c5-cc035599aa3b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationCostsOfGoodsSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-9e2e-76b7-be06-331d6ba4d415" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationCostsOfGoodsSold_019dab70-9e2e-70d3-94c5-cc035599aa3b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationGrossProfitLoss_019dab70-9e2e-7b15-995c-0ad01a32583b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationGrossProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-9e2e-76b7-be06-331d6ba4d415" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationGrossProfitLoss_019dab70-9e2e-7b15-995c-0ad01a32583b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationGeneralAndAdministrativeExpense_019dab70-9e2e-78da-bbf1-dffe5950511e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationGeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-9e2e-76b7-be06-331d6ba4d415" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationGeneralAndAdministrativeExpense_019dab70-9e2e-78da-bbf1-dffe5950511e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_DisposalGroupIncludingDiscontinuedOperationResearchAndDevelopmentExpense_019dab70-9e2e-7ac4-8c18-8eaabe48819e" xlink:href="mnk-20260331.xsd#mnk_DisposalGroupIncludingDiscontinuedOperationResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-9e2e-76b7-be06-331d6ba4d415" xlink:to="loc_mnk_DisposalGroupIncludingDiscontinuedOperationResearchAndDevelopmentExpense_019dab70-9e2e-7ac4-8c18-8eaabe48819e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingIncomeLoss_019dab70-9e2e-752f-8699-e6c3da2c027a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-9e2e-76b7-be06-331d6ba4d415" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationOperatingIncomeLoss_019dab70-9e2e-752f-8699-e6c3da2c027a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationInterestExpense_019dab70-9e2e-7d8b-b5fa-ba94767d616d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationInterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-9e2e-76b7-be06-331d6ba4d415" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationInterestExpense_019dab70-9e2e-7d8b-b5fa-ba94767d616d" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationInterestIncome_019dab70-9e2e-7302-94a9-a9e472d7f673" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupIncludingDiscontinuedOperationInterestIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-9e2e-76b7-be06-331d6ba4d415" xlink:to="loc_us-gaap_DisposalGroupIncludingDiscontinuedOperationInterestIncome_019dab70-9e2e-7302-94a9-a9e472d7f673" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_DisposalGroupIncludingDiscontinuedOperationOtherIncomeExpenseNet_019dab70-9e2e-7cd8-97cb-657cf71ae1eb" xlink:href="mnk-20260331.xsd#mnk_DisposalGroupIncludingDiscontinuedOperationOtherIncomeExpenseNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-9e2e-76b7-be06-331d6ba4d415" xlink:to="loc_mnk_DisposalGroupIncludingDiscontinuedOperationOtherIncomeExpenseNet_019dab70-9e2e-7cd8-97cb-657cf71ae1eb" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationBeforeIncomeTax_019dab70-9e2e-7494-b550-70b586db6df3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationBeforeIncomeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-9e2e-76b7-be06-331d6ba4d415" xlink:to="loc_us-gaap_DiscontinuedOperationIncomeLossFromDiscontinuedOperationBeforeIncomeTax_019dab70-9e2e-7494-b550-70b586db6df3" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationTaxEffectOfDiscontinuedOperation_019dab70-9e2e-7d2a-badc-d6835764b88c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DiscontinuedOperationTaxEffectOfDiscontinuedOperation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-9e2e-76b7-be06-331d6ba4d415" xlink:to="loc_us-gaap_DiscontinuedOperationTaxEffectOfDiscontinuedOperation_019dab70-9e2e-7d2a-badc-d6835764b88c" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax_019dab70-9e2e-7a85-8d05-4973748dd04a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-9e2e-76b7-be06-331d6ba4d415" xlink:to="loc_us-gaap_IncomeLossFromDiscontinuedOperationsNetOfTax_019dab70-9e2e-7a85-8d05-4973748dd04a" xlink:type="arc" order="11"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/DivestituresNarrativeDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#DivestituresNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/DivestituresNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-7e55-7cb4-af2f-6cb4279796ef" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_019dab70-7e55-79ff-9c1e-69e7adae3ce1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-7e55-7cb4-af2f-6cb4279796ef" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_019dab70-7e55-79ff-9c1e-69e7adae3ce1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationAxis_019dab70-7e55-7a8d-bb19-267ea8f97441" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_019dab70-7e55-79ff-9c1e-69e7adae3ce1" xlink:to="loc_us-gaap_DisposalGroupClassificationAxis_019dab70-7e55-7a8d-bb19-267ea8f97441" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_019dab70-7e55-7a8d-bb19-267ea8f97441_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_019dab70-7e55-7a8d-bb19-267ea8f97441" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_019dab70-7e55-7a8d-bb19-267ea8f97441_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupClassificationDomain_019dab70-7e55-72b7-ae1f-2dfc38d2ba28" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupClassificationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DisposalGroupClassificationAxis_019dab70-7e55-7a8d-bb19-267ea8f97441" xlink:to="loc_us-gaap_DisposalGroupClassificationDomain_019dab70-7e55-72b7-ae1f-2dfc38d2ba28" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupDisposedOfBySaleNotDiscontinuedOperationsMember_019dab70-7e55-7206-8e4b-36fa249fcd8c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupDisposedOfBySaleNotDiscontinuedOperationsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupClassificationDomain_019dab70-7e55-72b7-ae1f-2dfc38d2ba28" xlink:to="loc_us-gaap_DisposalGroupDisposedOfBySaleNotDiscontinuedOperationsMember_019dab70-7e55-7206-8e4b-36fa249fcd8c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019dab70-7e55-723f-980e-980b93975b76" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsTable_019dab70-7e55-79ff-9c1e-69e7adae3ce1" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019dab70-7e55-723f-980e-980b93975b76" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019dab70-7e55-723f-980e-980b93975b76_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019dab70-7e55-723f-980e-980b93975b76" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019dab70-7e55-723f-980e-980b93975b76_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019dab70-7e55-703d-92f5-db2949055eba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019dab70-7e55-723f-980e-980b93975b76" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019dab70-7e55-703d-92f5-db2949055eba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_ParHealthMember_019e0233-d5b6-792e-9b89-627056372fcf" xlink:href="mnk-20260331.xsd#mnk_ParHealthMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019dab70-7e55-703d-92f5-db2949055eba" xlink:to="loc_mnk_ParHealthMember_019e0233-d5b6-792e-9b89-627056372fcf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_TherakosDivestitureMember_019dab70-7e55-7cd9-bae6-3e66a19e3669" xlink:href="mnk-20260331.xsd#mnk_TherakosDivestitureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019dab70-7e55-703d-92f5-db2949055eba" xlink:to="loc_mnk_TherakosDivestitureMember_019dab70-7e55-7cd9-bae6-3e66a19e3669" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DiscontinuedOperationAmountOfContinuingCashFlowsAfterDisposal_019e1c9c-6b94-7313-b49c-f35adcb21604" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DiscontinuedOperationAmountOfContinuingCashFlowsAfterDisposal"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-7e55-7cb4-af2f-6cb4279796ef" xlink:to="loc_us-gaap_DiscontinuedOperationAmountOfContinuingCashFlowsAfterDisposal_019e1c9c-6b94-7313-b49c-f35adcb21604" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_TransitionServicesAgreementReceivable_019e166d-ff3b-7d82-be6d-4103bbfc608a" xlink:href="mnk-20260331.xsd#mnk_TransitionServicesAgreementReceivable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-7e55-7cb4-af2f-6cb4279796ef" xlink:to="loc_mnk_TransitionServicesAgreementReceivable_019e166d-ff3b-7d82-be6d-4103bbfc608a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_TransitionServicesAgreementPayable_019e166d-ff3b-7c2b-b8c4-21fae1abb533" xlink:href="mnk-20260331.xsd#mnk_TransitionServicesAgreementPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-7e55-7cb4-af2f-6cb4279796ef" xlink:to="loc_mnk_TransitionServicesAgreementPayable_019e166d-ff3b-7c2b-b8c4-21fae1abb533" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_PaymentsForDivestiture_019e023e-b113-7838-ad6c-e26e79c9b5db" xlink:href="mnk-20260331.xsd#mnk_PaymentsForDivestiture"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-7e55-7cb4-af2f-6cb4279796ef" xlink:to="loc_mnk_PaymentsForDivestiture_019e023e-b113-7838-ad6c-e26e79c9b5db" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_TransitionServicesAgreementTerm_019dab70-7e55-7208-9668-24fb5c2d60da" xlink:href="mnk-20260331.xsd#mnk_TransitionServicesAgreementTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-7e55-7cb4-af2f-6cb4279796ef" xlink:to="loc_mnk_TransitionServicesAgreementTerm_019dab70-7e55-7208-9668-24fb5c2d60da" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_TransitionServicesAgreementRevenue_019dab70-7e55-7a09-b446-528504f313c5" xlink:href="mnk-20260331.xsd#mnk_TransitionServicesAgreementRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsLineItems_019dab70-7e55-7cb4-af2f-6cb4279796ef" xlink:to="loc_mnk_TransitionServicesAgreementRevenue_019dab70-7e55-7a09-b446-528504f313c5" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/RevenuefromContractswithCustomersRollforwardofVariableConsiderationReservesDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#RevenuefromContractswithCustomersRollforwardofVariableConsiderationReservesDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/RevenuefromContractswithCustomersRollforwardofVariableConsiderationReservesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019cd357-73c2-70e8-8385-beeee8b44939" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019cd357-73c2-7c07-96ec-eab0ed498362" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019cd357-73c2-70e8-8385-beeee8b44939" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019cd357-73c2-7c07-96ec-eab0ed498362" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_VariableConsiderationAxis_019cd357-73c2-7bca-811a-814f9836568b" xlink:href="mnk-20260331.xsd#mnk_VariableConsiderationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019cd357-73c2-7c07-96ec-eab0ed498362" xlink:to="loc_mnk_VariableConsiderationAxis_019cd357-73c2-7bca-811a-814f9836568b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_VariableConsiderationDomain_019cd357-73c2-7bca-811a-814f9836568b_default" xlink:href="mnk-20260331.xsd#mnk_VariableConsiderationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_mnk_VariableConsiderationAxis_019cd357-73c2-7bca-811a-814f9836568b" xlink:to="loc_mnk_VariableConsiderationDomain_019cd357-73c2-7bca-811a-814f9836568b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_VariableConsiderationDomain_019cd357-73c2-7993-b364-2d3cb9e5bda3" xlink:href="mnk-20260331.xsd#mnk_VariableConsiderationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_mnk_VariableConsiderationAxis_019cd357-73c2-7bca-811a-814f9836568b" xlink:to="loc_mnk_VariableConsiderationDomain_019cd357-73c2-7993-b364-2d3cb9e5bda3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_RebatesandChargebacksMember_019cd357-73c2-77cd-9e3e-3466a61ce3d5" xlink:href="mnk-20260331.xsd#mnk_RebatesandChargebacksMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_VariableConsiderationDomain_019cd357-73c2-7993-b364-2d3cb9e5bda3" xlink:to="loc_mnk_RebatesandChargebacksMember_019cd357-73c2-77cd-9e3e-3466a61ce3d5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesReturnsAndAllowancesMember_019cd357-73c2-71b9-8ba3-acb9e2408c66" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SalesReturnsAndAllowancesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_VariableConsiderationDomain_019cd357-73c2-7993-b364-2d3cb9e5bda3" xlink:to="loc_us-gaap_SalesReturnsAndAllowancesMember_019cd357-73c2-71b9-8ba3-acb9e2408c66" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_OtherSalesDeductionsMember_019cd357-73c2-7129-a347-f64107e41ec0" xlink:href="mnk-20260331.xsd#mnk_OtherSalesDeductionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_VariableConsiderationDomain_019cd357-73c2-7993-b364-2d3cb9e5bda3" xlink:to="loc_mnk_OtherSalesDeductionsMember_019cd357-73c2-7129-a347-f64107e41ec0" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_MovementInContractWithCustomerLiabilityRollForward_019cd357-73c2-78f7-9e98-66e7b8579afc" xlink:href="mnk-20260331.xsd#mnk_MovementInContractWithCustomerLiabilityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019cd357-73c2-70e8-8385-beeee8b44939" xlink:to="loc_mnk_MovementInContractWithCustomerLiabilityRollForward_019cd357-73c2-78f7-9e98-66e7b8579afc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiability_019cd357-73c2-7fb4-ac06-66c86d5b8b44" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_MovementInContractWithCustomerLiabilityRollForward_019cd357-73c2-78f7-9e98-66e7b8579afc" xlink:to="loc_us-gaap_ContractWithCustomerLiability_019cd357-73c2-7fb4-ac06-66c86d5b8b44" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCumulativeCatchUpAdjustmentToRevenueChangeInEstimateOfTransactionPrice_019cd357-73c2-7f12-b2d3-4b37392a12ba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiabilityCumulativeCatchUpAdjustmentToRevenueChangeInEstimateOfTransactionPrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_MovementInContractWithCustomerLiabilityRollForward_019cd357-73c2-78f7-9e98-66e7b8579afc" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCumulativeCatchUpAdjustmentToRevenueChangeInEstimateOfTransactionPrice_019cd357-73c2-7f12-b2d3-4b37392a12ba" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_ContractWithCustomerLiabilityRevenueRecognizedIncludingOpeningBalance_019cd357-73c2-7c99-8512-bf9d75b82277" xlink:href="mnk-20260331.xsd#mnk_ContractWithCustomerLiabilityRevenueRecognizedIncludingOpeningBalance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_MovementInContractWithCustomerLiabilityRollForward_019cd357-73c2-78f7-9e98-66e7b8579afc" xlink:to="loc_mnk_ContractWithCustomerLiabilityRevenueRecognizedIncludingOpeningBalance_019cd357-73c2-7c99-8512-bf9d75b82277" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiability_019cd357-73c2-7e08-bca1-1a0b16ccacfa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiability"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/RevenuefromContractswithCustomersDisaggregationofRevenueDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#RevenuefromContractswithCustomersDisaggregationofRevenueDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/RevenuefromContractswithCustomersDisaggregationofRevenueDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019cd357-73c2-75a3-9d7e-024fe8c7b3f2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019cd357-73c2-738c-aaf7-be596731d7f0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019cd357-73c2-75a3-9d7e-024fe8c7b3f2" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019cd357-73c2-738c-aaf7-be596731d7f0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019cd357-73c2-7a3a-a9a9-d1958e81f58b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimingOfTransferOfGoodOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019cd357-73c2-738c-aaf7-be596731d7f0" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019cd357-73c2-7a3a-a9a9-d1958e81f58b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019cd357-73c2-7a3a-a9a9-d1958e81f58b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019cd357-73c2-7a3a-a9a9-d1958e81f58b" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019cd357-73c2-7a3a-a9a9-d1958e81f58b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019cd357-73c2-7192-beb1-0df8fc9a71aa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TimingOfTransferOfGoodOrServiceDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceAxis_019cd357-73c2-7a3a-a9a9-d1958e81f58b" xlink:to="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019cd357-73c2-7192-beb1-0df8fc9a71aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredAtPointInTimeMember_019cd357-73c2-7c17-a116-50f736f291f2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransferredAtPointInTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019cd357-73c2-7192-beb1-0df8fc9a71aa" xlink:to="loc_us-gaap_TransferredAtPointInTimeMember_019cd357-73c2-7c17-a116-50f736f291f2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransferredOverTimeMember_019cd357-73c2-7ecf-a3ca-519e6dcfcaf1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransferredOverTimeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TimingOfTransferOfGoodOrServiceDomain_019cd357-73c2-7192-beb1-0df8fc9a71aa" xlink:to="loc_us-gaap_TransferredOverTimeMember_019cd357-73c2-7ecf-a3ca-519e6dcfcaf1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_RevenueFromContractWithCustomerIncludingAssessedTaxPercentage_019cd357-73c2-785f-a989-c34174e18c41" xlink:href="mnk-20260331.xsd#mnk_RevenueFromContractWithCustomerIncludingAssessedTaxPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019cd357-73c2-75a3-9d7e-024fe8c7b3f2" xlink:to="loc_mnk_RevenueFromContractWithCustomerIncludingAssessedTaxPercentage_019cd357-73c2-785f-a989-c34174e18c41" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/RevenuefromContractswithCustomersFuturePerformanceObligationsDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#RevenuefromContractswithCustomersFuturePerformanceObligationsDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/RevenuefromContractswithCustomersFuturePerformanceObligationsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019cd357-73c2-7af4-9511-2fbda06a91dc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019cd357-73c2-7ffc-ad69-73bf4f7ec3e7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019cd357-73c2-7af4-9511-2fbda06a91dc" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019cd357-73c2-7ffc-ad69-73bf4f7ec3e7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_019cd357-73c2-7cd1-b1a1-a46a31788718" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019cd357-73c2-7ffc-ad69-73bf4f7ec3e7" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_019cd357-73c2-7cd1-b1a1-a46a31788718" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_019cd357-73c2-7d37-a79f-99e981e1dcee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019cd357-73c2-7af4-9511-2fbda06a91dc" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_019cd357-73c2-7d37-a79f-99e981e1dcee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019cd357-73c2-7c3f-b8a8-5a9b2cfc8d2b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019cd357-73c2-7af4-9511-2fbda06a91dc" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019cd357-73c2-7c3f-b8a8-5a9b2cfc8d2b" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/RevenuefromContractswithCustomersFuturePerformanceObligationsDetails_1" xlink:type="simple" xlink:href="mnk-20260331.xsd#RevenuefromContractswithCustomersFuturePerformanceObligationsDetails_1"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/RevenuefromContractswithCustomersFuturePerformanceObligationsDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019cd357-73c2-7af4-9511-2fbda06a91dc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019cd357-73c2-7ffc-ad69-73bf4f7ec3e7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019cd357-73c2-7af4-9511-2fbda06a91dc" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019cd357-73c2-7ffc-ad69-73bf4f7ec3e7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_019cd357-73c2-7d37-a79f-99e981e1dcee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019cd357-73c2-7af4-9511-2fbda06a91dc" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_019cd357-73c2-7d37-a79f-99e981e1dcee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019cd357-73c2-7c3f-b8a8-5a9b2cfc8d2b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019cd357-73c2-7af4-9511-2fbda06a91dc" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019cd357-73c2-7c3f-b8a8-5a9b2cfc8d2b" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/IncomeTaxesDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#IncomeTaxesDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/IncomeTaxesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_mnk_IncomeTaxesLineItems_019cd357-73c2-75d6-9b19-bf3c8780ccff" xlink:href="mnk-20260331.xsd#mnk_IncomeTaxesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_IncomeTaxesTable_019cd357-73c2-72ab-96a3-7bdc98842bc0" xlink:href="mnk-20260331.xsd#mnk_IncomeTaxesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_mnk_IncomeTaxesLineItems_019cd357-73c2-75d6-9b19-bf3c8780ccff" xlink:to="loc_mnk_IncomeTaxesTable_019cd357-73c2-72ab-96a3-7bdc98842bc0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019cd357-73c2-7d25-a528-4f2d0129ad51" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_mnk_IncomeTaxesTable_019cd357-73c2-72ab-96a3-7bdc98842bc0" xlink:to="loc_srt_StatementGeographicalAxis_019cd357-73c2-7d25-a528-4f2d0129ad51" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019cd357-73c2-7d25-a528-4f2d0129ad51_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019cd357-73c2-7d25-a528-4f2d0129ad51" xlink:to="loc_srt_SegmentGeographicalDomain_019cd357-73c2-7d25-a528-4f2d0129ad51_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019cd357-73c2-7cea-8459-c010cdb7cca9" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019cd357-73c2-7d25-a528-4f2d0129ad51" xlink:to="loc_srt_SegmentGeographicalDomain_019cd357-73c2-7cea-8459-c010cdb7cca9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_IE_019cd357-73c2-7bc3-8fce-0b75c82ff3f3" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_IE"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019cd357-73c2-7cea-8459-c010cdb7cca9" xlink:to="loc_country_IE_019cd357-73c2-7bc3-8fce-0b75c82ff3f3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019cd357-73c2-7509-824e-107f0db506c7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_IncomeTaxesLineItems_019cd357-73c2-75d6-9b19-bf3c8780ccff" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019cd357-73c2-7509-824e-107f0db506c7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019cd357-73c2-79bc-954a-016f4a231845" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_IncomeTaxesLineItems_019cd357-73c2-75d6-9b19-bf3c8780ccff" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesExtraordinaryItemsNoncontrollingInterest_019cd357-73c2-79bc-954a-016f4a231845" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_019cd357-73c2-7b71-818e-f1552b740554" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_IncomeTaxesLineItems_019cd357-73c2-75d6-9b19-bf3c8780ccff" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_019cd357-73c2-7b71-818e-f1552b740554" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_019cd357-73c2-7543-8fd1-457259834667" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_IncomeTaxesLineItems_019cd357-73c2-75d6-9b19-bf3c8780ccff" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationAtFederalStatutoryIncomeTaxRate_019cd357-73c2-7543-8fd1-457259834667" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefits_019cd357-73c2-76f7-a1f3-868528f94d75" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrecognizedTaxBenefits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_IncomeTaxesLineItems_019cd357-73c2-75d6-9b19-bf3c8780ccff" xlink:to="loc_us-gaap_UnrecognizedTaxBenefits_019cd357-73c2-76f7-a1f3-868528f94d75" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/InventoriesDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#InventoriesDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/InventoriesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryLineItems_019daac8-43d4-7e90-bc49-1063aa8898fe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryCurrentTable_019daac8-43d3-7307-bb42-7f32bca5bf81" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryCurrentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_InventoryLineItems_019daac8-43d4-7e90-bc49-1063aa8898fe" xlink:to="loc_us-gaap_InventoryCurrentTable_019daac8-43d3-7307-bb42-7f32bca5bf81" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_019daac8-43d3-7ab3-86ed-d49ef6dec96d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_InventoryCurrentTable_019daac8-43d3-7307-bb42-7f32bca5bf81" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_019daac8-43d3-7ab3-86ed-d49ef6dec96d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_019daac8-43d3-7ab3-86ed-d49ef6dec96d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_019daac8-43d3-7ab3-86ed-d49ef6dec96d" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_019daac8-43d3-7ab3-86ed-d49ef6dec96d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_019daac8-43d3-7dbd-811b-253e4d370f99" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_019daac8-43d3-7ab3-86ed-d49ef6dec96d" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_019daac8-43d3-7dbd-811b-253e4d370f99" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_InventoryNetMember_019daac8-43d3-7a9f-b338-2df9b146c038" xlink:href="mnk-20260331.xsd#mnk_InventoryNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_019daac8-43d3-7dbd-811b-253e4d370f99" xlink:to="loc_mnk_InventoryNetMember_019daac8-43d3-7a9f-b338-2df9b146c038" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_InventoryNoncurrentMember_019daac8-43d3-7f48-a7ca-e4fc79766ad7" xlink:href="mnk-20260331.xsd#mnk_InventoryNoncurrentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_019daac8-43d3-7dbd-811b-253e4d370f99" xlink:to="loc_mnk_InventoryNoncurrentMember_019daac8-43d3-7f48-a7ca-e4fc79766ad7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryRawMaterialsAndPurchasedPartsNetOfReserves_019cd357-73c2-7b91-9cd5-1c2a4137ea5f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryRawMaterialsAndPurchasedPartsNetOfReserves"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InventoryLineItems_019daac8-43d4-7e90-bc49-1063aa8898fe" xlink:to="loc_us-gaap_InventoryRawMaterialsAndPurchasedPartsNetOfReserves_019cd357-73c2-7b91-9cd5-1c2a4137ea5f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryWorkInProcessNetOfReserves_019cd357-73c2-7edb-9631-f6da56ac6963" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryWorkInProcessNetOfReserves"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InventoryLineItems_019daac8-43d4-7e90-bc49-1063aa8898fe" xlink:to="loc_us-gaap_InventoryWorkInProcessNetOfReserves_019cd357-73c2-7edb-9631-f6da56ac6963" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryFinishedGoodsNetOfReserves_019cd357-73c2-7e51-af0f-dfc40ed51ed5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryFinishedGoodsNetOfReserves"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InventoryLineItems_019daac8-43d4-7e90-bc49-1063aa8898fe" xlink:to="loc_us-gaap_InventoryFinishedGoodsNetOfReserves_019cd357-73c2-7e51-af0f-dfc40ed51ed5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryNet_019cd357-73c2-7494-89c5-0b5046ece058" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InventoryLineItems_019daac8-43d4-7e90-bc49-1063aa8898fe" xlink:to="loc_us-gaap_InventoryNet_019cd357-73c2-7494-89c5-0b5046ece058" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryNoncurrent_019dabc7-5d50-71a6-a204-920be9eb2dcd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InventoryLineItems_019daac8-43d4-7e90-bc49-1063aa8898fe" xlink:to="loc_us-gaap_InventoryNoncurrent_019dabc7-5d50-71a6-a204-920be9eb2dcd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryAdjustments_019daac8-43d4-7729-9f23-1d245be392a7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_InventoryLineItems_019daac8-43d4-7e90-bc49-1063aa8898fe" xlink:to="loc_us-gaap_InventoryAdjustments_019daac8-43d4-7729-9f23-1d245be392a7" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/PropertyPlantandEquipmentScheduleofPropertyPlantandEquipmentDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#PropertyPlantandEquipmentScheduleofPropertyPlantandEquipmentDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/PropertyPlantandEquipmentScheduleofPropertyPlantandEquipmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019daacb-2873-71aa-a2c9-167028685f01" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019daacb-2873-70d6-80d4-bf05ffe9e151" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019daacb-2873-71aa-a2c9-167028685f01" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019daacb-2873-70d6-80d4-bf05ffe9e151" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019daacb-2873-77e5-b60b-1926358c18c0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019daacb-2873-70d6-80d4-bf05ffe9e151" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019daacb-2873-77e5-b60b-1926358c18c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019daacb-2873-77e5-b60b-1926358c18c0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019daacb-2873-77e5-b60b-1926358c18c0" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019daacb-2873-77e5-b60b-1926358c18c0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019daacb-2873-7193-ac0e-e3103f36660d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019daacb-2873-77e5-b60b-1926358c18c0" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019daacb-2873-7193-ac0e-e3103f36660d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LandMember_019daacb-2873-70ce-be30-e599b49ccd39" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LandMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019daacb-2873-7193-ac0e-e3103f36660d" xlink:to="loc_us-gaap_LandMember_019daacb-2873-70ce-be30-e599b49ccd39" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BuildingMember_019daacb-2873-7476-83cc-e5321846ffbe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BuildingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019daacb-2873-7193-ac0e-e3103f36660d" xlink:to="loc_us-gaap_BuildingMember_019daacb-2873-7476-83cc-e5321846ffbe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_019daacb-2873-7196-bc9d-4940a278dd18" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SoftwareAndSoftwareDevelopmentCostsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019daacb-2873-7193-ac0e-e3103f36660d" xlink:to="loc_us-gaap_SoftwareAndSoftwareDevelopmentCostsMember_019daacb-2873-7196-bc9d-4940a278dd18" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MachineryAndEquipmentMember_019daacb-2873-7b41-b939-5781d1ece5ae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_MachineryAndEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019daacb-2873-7193-ac0e-e3103f36660d" xlink:to="loc_us-gaap_MachineryAndEquipmentMember_019daacb-2873-7b41-b939-5781d1ece5ae" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConstructionInProgressMember_019daacb-2873-7208-9f68-0ed72c7ffe92" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConstructionInProgressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019daacb-2873-7193-ac0e-e3103f36660d" xlink:to="loc_us-gaap_ConstructionInProgressMember_019daacb-2873-7208-9f68-0ed72c7ffe92" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_019daacb-2873-73b8-8e7b-d42be056a8ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019daacb-2873-71aa-a2c9-167028685f01" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_019daacb-2873-73b8-8e7b-d42be056a8ab" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_019daacb-2873-7f8c-af02-f4742acdc243" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019daacb-2873-71aa-a2c9-167028685f01" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_019daacb-2873-7f8c-af02-f4742acdc243" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_019daacb-2873-7661-95aa-45b7f096bd4a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019daacb-2873-71aa-a2c9-167028685f01" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_019daacb-2873-7661-95aa-45b7f096bd4a" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/GoodwillandIntangibleAssetsScheduleOfIntangibleAssetsDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#GoodwillandIntangibleAssetsScheduleOfIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/GoodwillandIntangibleAssetsScheduleOfIntangibleAssetsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_mnk_ScheduleofIntangibleAssetbyMajorClassLineItems_019cd357-73c2-79ad-8e15-10e4bc824f69" xlink:href="mnk-20260331.xsd#mnk_ScheduleofIntangibleAssetbyMajorClassLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_019cd357-73c2-79a0-a9f5-44e411f3999f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_mnk_ScheduleofIntangibleAssetbyMajorClassLineItems_019cd357-73c2-79ad-8e15-10e4bc824f69" xlink:to="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_019cd357-73c2-79a0-a9f5-44e411f3999f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019cd357-73c2-72f8-aa33-519a0ae4dd63" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAcquiredFiniteLivedIntangibleAssetByMajorClassTable_019cd357-73c2-79a0-a9f5-44e411f3999f" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019cd357-73c2-72f8-aa33-519a0ae4dd63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019cd357-73c2-72f8-aa33-519a0ae4dd63_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019cd357-73c2-72f8-aa33-519a0ae4dd63" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019cd357-73c2-72f8-aa33-519a0ae4dd63_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019cd357-73c2-7b7c-8cbd-b4bb02b173b8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019cd357-73c2-72f8-aa33-519a0ae4dd63" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019cd357-73c2-7b7c-8cbd-b4bb02b173b8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_DevelopedTechnologyMember_019dbf59-a909-78ce-a86f-ce3cfec491a4" xlink:href="mnk-20260331.xsd#mnk_DevelopedTechnologyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019cd357-73c2-7b7c-8cbd-b4bb02b173b8" xlink:to="loc_mnk_DevelopedTechnologyMember_019dbf59-a909-78ce-a86f-ce3cfec491a4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019cd357-73c2-7b81-9972-47069ba1de9d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_ScheduleofIntangibleAssetbyMajorClassLineItems_019cd357-73c2-79ad-8e15-10e4bc824f69" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019cd357-73c2-7b81-9972-47069ba1de9d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019cd357-73c2-77c7-8fc0-df88283fc0b9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_ScheduleofIntangibleAssetbyMajorClassLineItems_019cd357-73c2-79ad-8e15-10e4bc824f69" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019cd357-73c2-77c7-8fc0-df88283fc0b9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019cd357-73c2-74e4-a158-4565fc1631a6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_ScheduleofIntangibleAssetbyMajorClassLineItems_019cd357-73c2-79ad-8e15-10e4bc824f69" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019cd357-73c2-74e4-a158-4565fc1631a6" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/DebtScheduleofDebtDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#DebtScheduleofDebtDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/DebtScheduleofDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019cd357-73c2-7cab-b694-eaea3d782659" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019cd357-73c2-7539-92c2-36caf44a9aab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cd357-73c2-7cab-b694-eaea3d782659" xlink:to="loc_us-gaap_DebtInstrumentTable_019cd357-73c2-7539-92c2-36caf44a9aab" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019cd357-73c2-717c-9b54-93f92e87dd9d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019cd357-73c2-7539-92c2-36caf44a9aab" xlink:to="loc_us-gaap_DebtInstrumentAxis_019cd357-73c2-717c-9b54-93f92e87dd9d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c2-717c-9b54-93f92e87dd9d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019cd357-73c2-717c-9b54-93f92e87dd9d" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c2-717c-9b54-93f92e87dd9d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c2-74d8-ad85-bc2e1e57c31e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019cd357-73c2-717c-9b54-93f92e87dd9d" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c2-74d8-ad85-bc2e1e57c31e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_TermLoanDueApril2031Member_019cd357-73c2-7f1c-bc7c-36155d1776e3" xlink:href="mnk-20260331.xsd#mnk_TermLoanDueApril2031Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c2-74d8-ad85-bc2e1e57c31e" xlink:to="loc_mnk_TermLoanDueApril2031Member_019cd357-73c2-7f1c-bc7c-36155d1776e3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_A8.50SeniorSecuredNotesDueApril2031Member_019cd357-73c2-7aa5-addd-3c8abbaff69f" xlink:href="mnk-20260331.xsd#mnk_A8.50SeniorSecuredNotesDueApril2031Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c2-74d8-ad85-bc2e1e57c31e" xlink:to="loc_mnk_A8.50SeniorSecuredNotesDueApril2031Member_019cd357-73c2-7aa5-addd-3c8abbaff69f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShortTermBorrowingsAbstract_019cd357-73c2-772e-8941-b68596901d59" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShortTermBorrowingsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cd357-73c2-7cab-b694-eaea3d782659" xlink:to="loc_us-gaap_ShortTermBorrowingsAbstract_019cd357-73c2-772e-8941-b68596901d59" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_DebtInstrumentFaceAmountCurrentMaturity_019cd357-73c2-796a-a15a-4ad3cc02fb0f" xlink:href="mnk-20260331.xsd#mnk_DebtInstrumentFaceAmountCurrentMaturity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermBorrowingsAbstract_019cd357-73c2-772e-8941-b68596901d59" xlink:to="loc_mnk_DebtInstrumentFaceAmountCurrentMaturity_019cd357-73c2-796a-a15a-4ad3cc02fb0f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_019cd357-73c2-7f9b-a2db-77590d2bd1b7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShortTermBorrowingsAbstract_019cd357-73c2-772e-8941-b68596901d59" xlink:to="loc_us-gaap_LongTermDebtCurrent_019cd357-73c2-7f9b-a2db-77590d2bd1b7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrentAbstract_019cd357-73c2-7c63-ad5f-24698e0aabba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtNoncurrentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cd357-73c2-7cab-b694-eaea3d782659" xlink:to="loc_us-gaap_LongTermDebtNoncurrentAbstract_019cd357-73c2-7c63-ad5f-24698e0aabba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_DebtInstrumentFaceAmountNoncurrentMaturity_019cd357-73c2-7257-bbc9-c72684d2d107" xlink:href="mnk-20260331.xsd#mnk_DebtInstrumentFaceAmountNoncurrentMaturity"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtNoncurrentAbstract_019cd357-73c2-7c63-ad5f-24698e0aabba" xlink:to="loc_mnk_DebtInstrumentFaceAmountNoncurrentMaturity_019cd357-73c2-7257-bbc9-c72684d2d107" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019cd357-73c2-7f58-9dee-06ac791abc0b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongTermDebtNoncurrentAbstract_019cd357-73c2-7c63-ad5f-24698e0aabba" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019cd357-73c2-7f58-9dee-06ac791abc0b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019cd357-73c2-79ad-9641-62a199d6efe6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cd357-73c2-7cab-b694-eaea3d782659" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019cd357-73c2-79ad-9641-62a199d6efe6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtLongtermAndShorttermCombinedAmount_019cd357-73c2-73b2-a838-e7afa7bd2d1d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtLongtermAndShorttermCombinedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cd357-73c2-7cab-b694-eaea3d782659" xlink:to="loc_us-gaap_DebtLongtermAndShorttermCombinedAmount_019cd357-73c2-73b2-a838-e7afa7bd2d1d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019dfd63-78a8-732d-9404-78b0b0aec787" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019cd357-73c2-7cab-b694-eaea3d782659" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019dfd63-78a8-732d-9404-78b0b0aec787" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/DebtScheduleofApplicableInterestRatesonVariablerateDebtDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#DebtScheduleofApplicableInterestRatesonVariablerateDebtDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/DebtScheduleofApplicableInterestRatesonVariablerateDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_mnk_ScheduleofApplicableInterestRateonVariablerateDebtLineItems_019cd357-73c2-7387-acfa-15bd9826ea68" xlink:href="mnk-20260331.xsd#mnk_ScheduleofApplicableInterestRateonVariablerateDebtLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_ScheduleofApplicableInterestRateonVariablerateDebtTable_019cd357-73c2-7205-9755-62ce4a5805f5" xlink:href="mnk-20260331.xsd#mnk_ScheduleofApplicableInterestRateonVariablerateDebtTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_mnk_ScheduleofApplicableInterestRateonVariablerateDebtLineItems_019cd357-73c2-7387-acfa-15bd9826ea68" xlink:to="loc_mnk_ScheduleofApplicableInterestRateonVariablerateDebtTable_019cd357-73c2-7205-9755-62ce4a5805f5" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019cd357-73c2-7bb0-ac9b-05549dbb6784" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_mnk_ScheduleofApplicableInterestRateonVariablerateDebtTable_019cd357-73c2-7205-9755-62ce4a5805f5" xlink:to="loc_us-gaap_DebtInstrumentAxis_019cd357-73c2-7bb0-ac9b-05549dbb6784" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c2-7bb0-ac9b-05549dbb6784_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019cd357-73c2-7bb0-ac9b-05549dbb6784" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c2-7bb0-ac9b-05549dbb6784_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c2-79b2-be4a-738b5d90e1ef" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019cd357-73c2-7bb0-ac9b-05549dbb6784" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c2-79b2-be4a-738b5d90e1ef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_FirstOutTakebackTermLoanDueNovember2028Member_019cd357-73c2-78d7-8f06-01c2603bb8f8" xlink:href="mnk-20260331.xsd#mnk_FirstOutTakebackTermLoanDueNovember2028Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c2-79b2-be4a-738b5d90e1ef" xlink:to="loc_mnk_FirstOutTakebackTermLoanDueNovember2028Member_019cd357-73c2-78d7-8f06-01c2603bb8f8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_A8.50SeniorSecuredNotesDueApril2031Member_019cd357-73c2-7569-b648-02e4dd14b4c7" xlink:href="mnk-20260331.xsd#mnk_A8.50SeniorSecuredNotesDueApril2031Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c2-79b2-be4a-738b5d90e1ef" xlink:to="loc_mnk_A8.50SeniorSecuredNotesDueApril2031Member_019cd357-73c2-7569-b648-02e4dd14b4c7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtWeightedAverageInterestRate_019cd357-73c2-76be-bc84-bfafa0a62990" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtWeightedAverageInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_ScheduleofApplicableInterestRateonVariablerateDebtLineItems_019cd357-73c2-7387-acfa-15bd9826ea68" xlink:to="loc_us-gaap_LongtermDebtWeightedAverageInterestRate_019cd357-73c2-76be-bc84-bfafa0a62990" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019dfd64-306e-7a13-b867-7637160e67f0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_ScheduleofApplicableInterestRateonVariablerateDebtLineItems_019cd357-73c2-7387-acfa-15bd9826ea68" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019dfd64-306e-7a13-b867-7637160e67f0" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/CommitmentsandContingenciesDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#CommitmentsandContingenciesDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/CommitmentsandContingenciesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_019daafd-b0e9-7b5d-8d71-99e4f29f3fb5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_019daafd-b0e9-7bd9-9e11-7fc3c10fc8d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_019daafd-b0e9-7b5d-8d71-99e4f29f3fb5" xlink:to="loc_us-gaap_LossContingenciesTable_019daafd-b0e9-7bd9-9e11-7fc3c10fc8d7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseAxis_019daafd-b0e9-7644-9b94-30911638ead5" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_LitigationCaseAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019daafd-b0e9-7bd9-9e11-7fc3c10fc8d7" xlink:to="loc_srt_LitigationCaseAxis_019daafd-b0e9-7644-9b94-30911638ead5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_019daafd-b0e9-7644-9b94-30911638ead5_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_LitigationCaseAxis_019daafd-b0e9-7644-9b94-30911638ead5" xlink:to="loc_srt_LitigationCaseTypeDomain_019daafd-b0e9-7644-9b94-30911638ead5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_019daafd-b0e9-7f18-9bb4-efe8e38b7a94" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_LitigationCaseAxis_019daafd-b0e9-7644-9b94-30911638ead5" xlink:to="loc_srt_LitigationCaseTypeDomain_019daafd-b0e9-7f18-9bb4-efe8e38b7a94" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_EndoBankruptcyMember_019daafd-b0e9-7c28-a7bb-49911db67caf" xlink:href="mnk-20260331.xsd#mnk_EndoBankruptcyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_019daafd-b0e9-7f18-9bb4-efe8e38b7a94" xlink:to="loc_mnk_EndoBankruptcyMember_019daafd-b0e9-7c28-a7bb-49911db67caf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_MallinckrodtPharmaceuticalsIrelandLimitedV.s.AirgasTherapeuticsLLCMember_019daafd-b0e9-7b93-965c-f477d14ae230" xlink:href="mnk-20260331.xsd#mnk_MallinckrodtPharmaceuticalsIrelandLimitedV.s.AirgasTherapeuticsLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_019daafd-b0e9-7f18-9bb4-efe8e38b7a94" xlink:to="loc_mnk_MallinckrodtPharmaceuticalsIrelandLimitedV.s.AirgasTherapeuticsLLCMember_019daafd-b0e9-7b93-965c-f477d14ae230" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LitigationStatusAxis_019daafd-b0e9-740c-8ccf-5b8ecaa30d20" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LitigationStatusAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019daafd-b0e9-7bd9-9e11-7fc3c10fc8d7" xlink:to="loc_us-gaap_LitigationStatusAxis_019daafd-b0e9-740c-8ccf-5b8ecaa30d20" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LitigationStatusDomain_019daafd-b0e9-740c-8ccf-5b8ecaa30d20_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LitigationStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LitigationStatusAxis_019daafd-b0e9-740c-8ccf-5b8ecaa30d20" xlink:to="loc_us-gaap_LitigationStatusDomain_019daafd-b0e9-740c-8ccf-5b8ecaa30d20_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LitigationStatusDomain_019daafd-b0e9-742c-83bd-398de56c2e1d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LitigationStatusDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LitigationStatusAxis_019daafd-b0e9-740c-8ccf-5b8ecaa30d20" xlink:to="loc_us-gaap_LitigationStatusDomain_019daafd-b0e9-742c-83bd-398de56c2e1d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PendingLitigationMember_019daafd-b0e9-7449-a90e-37324cebca43" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PendingLitigationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LitigationStatusDomain_019daafd-b0e9-742c-83bd-398de56c2e1d" xlink:to="loc_us-gaap_PendingLitigationMember_019daafd-b0e9-7449-a90e-37324cebca43" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019daafd-b0e9-7cb7-a7cc-d071a6345a8e" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019daafd-b0e9-7bd9-9e11-7fc3c10fc8d7" xlink:to="loc_srt_RangeAxis_019daafd-b0e9-7cb7-a7cc-d071a6345a8e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019daafd-b0e9-7cb7-a7cc-d071a6345a8e_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019daafd-b0e9-7cb7-a7cc-d071a6345a8e" xlink:to="loc_srt_RangeMember_019daafd-b0e9-7cb7-a7cc-d071a6345a8e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019daafd-b0e9-7790-9696-5252d2960277" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019daafd-b0e9-7cb7-a7cc-d071a6345a8e" xlink:to="loc_srt_RangeMember_019daafd-b0e9-7790-9696-5252d2960277" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019daafd-b0e9-79b5-a0cf-4446ade840e5" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019daafd-b0e9-7790-9696-5252d2960277" xlink:to="loc_srt_MaximumMember_019daafd-b0e9-79b5-a0cf-4446ade840e5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_LossContingencyNumberOfAdditionalSubpoenas_019daafd-b0e9-7eb8-be3c-c86d7f3dfca5" xlink:href="mnk-20260331.xsd#mnk_LossContingencyNumberOfAdditionalSubpoenas"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019daafd-b0e9-7b5d-8d71-99e4f29f3fb5" xlink:to="loc_mnk_LossContingencyNumberOfAdditionalSubpoenas_019daafd-b0e9-7eb8-be3c-c86d7f3dfca5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyAccrualAtCarryingValue_019daafd-b0e9-7293-82c7-00a08ae2f27d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingencyAccrualAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019daafd-b0e9-7b5d-8d71-99e4f29f3fb5" xlink:to="loc_us-gaap_LossContingencyAccrualAtCarryingValue_019daafd-b0e9-7293-82c7-00a08ae2f27d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractualObligationFutureMinimumPaymentsDueRemainderOfFiscalYear_019daaff-8c99-7557-9ab5-684e1b088a71" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractualObligationFutureMinimumPaymentsDueRemainderOfFiscalYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019daafd-b0e9-7b5d-8d71-99e4f29f3fb5" xlink:to="loc_us-gaap_ContractualObligationFutureMinimumPaymentsDueRemainderOfFiscalYear_019daaff-8c99-7557-9ab5-684e1b088a71" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractualObligationDueInNextTwelveMonths_019daafd-b0e9-7fe8-97db-40130a6254fd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractualObligationDueInNextTwelveMonths"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019daafd-b0e9-7b5d-8d71-99e4f29f3fb5" xlink:to="loc_us-gaap_ContractualObligationDueInNextTwelveMonths_019daafd-b0e9-7fe8-97db-40130a6254fd" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractualObligationDueInSecondYear_019daafd-b0e9-76b2-9467-05901fde7e84" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractualObligationDueInSecondYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019daafd-b0e9-7b5d-8d71-99e4f29f3fb5" xlink:to="loc_us-gaap_ContractualObligationDueInSecondYear_019daafd-b0e9-76b2-9467-05901fde7e84" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractualObligationDueInThirdYear_019daafd-b0e9-789f-b128-8a317abd51b9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractualObligationDueInThirdYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019daafd-b0e9-7b5d-8d71-99e4f29f3fb5" xlink:to="loc_us-gaap_ContractualObligationDueInThirdYear_019daafd-b0e9-789f-b128-8a317abd51b9" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractualObligationDueInFourthYear_019daafd-b0e9-747f-ae34-7f186d23d70f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractualObligationDueInFourthYear"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019daafd-b0e9-7b5d-8d71-99e4f29f3fb5" xlink:to="loc_us-gaap_ContractualObligationDueInFourthYear_019daafd-b0e9-747f-ae34-7f186d23d70f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractualObligation_019daafd-b0e9-7d68-8550-b278b16a8c43" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractualObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019daafd-b0e9-7b5d-8d71-99e4f29f3fb5" xlink:to="loc_us-gaap_ContractualObligation_019daafd-b0e9-7d68-8550-b278b16a8c43" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_ContractualObligationContingentConsideration_019dbf23-222b-7529-aee8-709beda2a79c" xlink:href="mnk-20260331.xsd#mnk_ContractualObligationContingentConsideration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019daafd-b0e9-7b5d-8d71-99e4f29f3fb5" xlink:to="loc_mnk_ContractualObligationContingentConsideration_019dbf23-222b-7529-aee8-709beda2a79c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_LossContingencyNumberOfPatentsAsserted_019daafd-b0e9-7ef4-be43-a07aecb354e1" xlink:href="mnk-20260331.xsd#mnk_LossContingencyNumberOfPatentsAsserted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019daafd-b0e9-7b5d-8d71-99e4f29f3fb5" xlink:to="loc_mnk_LossContingencyNumberOfPatentsAsserted_019daafd-b0e9-7ef4-be43-a07aecb354e1" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyDamagesAwardedValue_019daafd-b0e9-7dd6-af62-9262fb24afc1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingencyDamagesAwardedValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019daafd-b0e9-7b5d-8d71-99e4f29f3fb5" xlink:to="loc_us-gaap_LossContingencyDamagesAwardedValue_019daafd-b0e9-7dd6-af62-9262fb24afc1" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_LossContingencyNumberOfInvalidationTrials_019daafd-b0e9-7b2d-9bb3-2d5aeb9b3189" xlink:href="mnk-20260331.xsd#mnk_LossContingencyNumberOfInvalidationTrials"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019daafd-b0e9-7b5d-8d71-99e4f29f3fb5" xlink:to="loc_mnk_LossContingencyNumberOfInvalidationTrials_019daafd-b0e9-7b2d-9bb3-2d5aeb9b3189" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_LossContingencyNumberOfPTERegistrations_019daafd-b0e9-7339-98ab-7b741a46325c" xlink:href="mnk-20260331.xsd#mnk_LossContingencyNumberOfPTERegistrations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019daafd-b0e9-7b5d-8d71-99e4f29f3fb5" xlink:to="loc_mnk_LossContingencyNumberOfPTERegistrations_019daafd-b0e9-7339-98ab-7b741a46325c" xlink:type="arc" order="12"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofFairValueofAssetsandLiabilitiesMeasuredonaRecurringBasisDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#FinancialInstrumentsandFairValueMeasurementsScheduleofFairValueofAssetsandLiabilitiesMeasuredonaRecurringBasisDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofFairValueofAssetsandLiabilitiesMeasuredonaRecurringBasisDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019cd357-73c3-7eff-942b-710a556d0826" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019cd357-73c3-7a1c-ab7e-b6196a1c8fae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019cd357-73c3-7eff-942b-710a556d0826" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019cd357-73c3-7a1c-ab7e-b6196a1c8fae" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cd357-73c3-743b-aa3b-8ac8c83f5cab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019cd357-73c3-7a1c-ab7e-b6196a1c8fae" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cd357-73c3-743b-aa3b-8ac8c83f5cab" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-743b-aa3b-8ac8c83f5cab_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cd357-73c3-743b-aa3b-8ac8c83f5cab" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-743b-aa3b-8ac8c83f5cab_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-7fb2-8292-2ff8a905cb1a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cd357-73c3-743b-aa3b-8ac8c83f5cab" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-7fb2-8292-2ff8a905cb1a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019cd357-73c3-791b-bdba-e46ad924575e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-7fb2-8292-2ff8a905cb1a" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019cd357-73c3-791b-bdba-e46ad924575e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019cd357-73c3-762e-af72-3d1e8d30b094" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-7fb2-8292-2ff8a905cb1a" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019cd357-73c3-762e-af72-3d1e8d30b094" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019cd357-73c3-75f1-86c6-17b484a5ecf7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-7fb2-8292-2ff8a905cb1a" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019cd357-73c3-75f1-86c6-17b484a5ecf7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosureAbstract_019cd357-73c3-7b00-99e2-8a94a7a52a80" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019cd357-73c3-7eff-942b-710a556d0826" xlink:to="loc_us-gaap_AssetsFairValueDisclosureAbstract_019cd357-73c3-7b00-99e2-8a94a7a52a80" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsFairValueDisclosure_019cd357-73c3-7e9f-bddb-5b5705cc1c7e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherAssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019cd357-73c3-7b00-99e2-8a94a7a52a80" xlink:to="loc_us-gaap_OtherAssetsFairValueDisclosure_019cd357-73c3-7e9f-bddb-5b5705cc1c7e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_019cd357-73c3-74c8-a53a-992af02cfe4b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsFairValueDisclosureAbstract_019cd357-73c3-7b00-99e2-8a94a7a52a80" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_019cd357-73c3-74c8-a53a-992af02cfe4b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_019cd357-73c3-720d-bb75-0a42dbbbd60d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesFairValueDisclosureAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019cd357-73c3-7eff-942b-710a556d0826" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_019cd357-73c3-720d-bb75-0a42dbbbd60d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent_019cd357-73c3-713a-91a9-5fe7f2f3e771" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_019cd357-73c3-720d-bb75-0a42dbbbd60d" xlink:to="loc_us-gaap_DeferredCompensationLiabilityCurrentAndNoncurrent_019cd357-73c3-713a-91a9-5fe7f2f3e771" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_ContingentLiabilitiesFairValueDisclosure_019cd357-73c3-7165-891f-c3c6ac0bf77f" xlink:href="mnk-20260331.xsd#mnk_ContingentLiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_019cd357-73c3-720d-bb75-0a42dbbbd60d" xlink:to="loc_mnk_ContingentLiabilitiesFairValueDisclosure_019cd357-73c3-7165-891f-c3c6ac0bf77f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesFairValueDisclosure_019cd357-73c3-7252-b1dd-004df139ff46" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LiabilitiesFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesFairValueDisclosureAbstract_019cd357-73c3-720d-bb75-0a42dbbbd60d" xlink:to="loc_us-gaap_LiabilitiesFairValueDisclosure_019cd357-73c3-7252-b1dd-004df139ff46" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/FinancialInstrumentsandFairValueMeasurementsAdditionalInformationDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#FinancialInstrumentsandFairValueMeasurementsAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/FinancialInstrumentsandFairValueMeasurementsAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019dab17-c7e6-7d00-a0db-a882c900ebe2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019dab17-c7e6-752a-a477-0e1eea75a535" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019dab17-c7e6-7d00-a0db-a882c900ebe2" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019dab17-c7e6-752a-a477-0e1eea75a535" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GuaranteeObligationsByNatureAxis_019dab18-29a1-7ba5-82bf-028b414cadb8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GuaranteeObligationsByNatureAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019dab17-c7e6-752a-a477-0e1eea75a535" xlink:to="loc_us-gaap_GuaranteeObligationsByNatureAxis_019dab18-29a1-7ba5-82bf-028b414cadb8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GuaranteeObligationsNatureDomain_019dab18-29a1-7ba5-82bf-028b414cadb8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GuaranteeObligationsNatureDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_GuaranteeObligationsByNatureAxis_019dab18-29a1-7ba5-82bf-028b414cadb8" xlink:to="loc_us-gaap_GuaranteeObligationsNatureDomain_019dab18-29a1-7ba5-82bf-028b414cadb8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GuaranteeObligationsNatureDomain_019dab18-29a1-7f64-9c0a-6e23242ca87c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GuaranteeObligationsNatureDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_GuaranteeObligationsByNatureAxis_019dab18-29a1-7ba5-82bf-028b414cadb8" xlink:to="loc_us-gaap_GuaranteeObligationsNatureDomain_019dab18-29a1-7f64-9c0a-6e23242ca87c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IndemnificationGuaranteeMember_019dab18-29a1-7e6a-9dcc-c9769af45379" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IndemnificationGuaranteeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GuaranteeObligationsNatureDomain_019dab18-29a1-7f64-9c0a-6e23242ca87c" xlink:to="loc_us-gaap_IndemnificationGuaranteeMember_019dab18-29a1-7e6a-9dcc-c9769af45379" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019dab18-29a1-71a5-beff-04b7dab1c529" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019dab17-c7e6-752a-a477-0e1eea75a535" xlink:to="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019dab18-29a1-71a5-beff-04b7dab1c529" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019dab18-29a1-71a5-beff-04b7dab1c529_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019dab18-29a1-71a5-beff-04b7dab1c529" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019dab18-29a1-71a5-beff-04b7dab1c529_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019dab18-29a1-7652-9e6d-d94ee0bd9720" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementBalanceSheetAndAdditionalDisclosuresByDisposalGroupsIncludingDiscontinuedOperationsAxis_019dab18-29a1-71a5-beff-04b7dab1c529" xlink:to="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019dab18-29a1-7652-9e6d-d94ee0bd9720" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_MallinckrodtBakerMember_019dab18-29a1-7b4d-b58f-09c16389830f" xlink:href="mnk-20260331.xsd#mnk_MallinckrodtBakerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisposalGroupsIncludingDiscontinuedOperationsNameDomain_019dab18-29a1-7652-9e6d-d94ee0bd9720" xlink:to="loc_mnk_MallinckrodtBakerMember_019dab18-29a1-7b4d-b58f-09c16389830f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019dd484-1f4c-7a78-ad72-6d6a2fa4ba20" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019dab17-c7e6-752a-a477-0e1eea75a535" xlink:to="loc_us-gaap_DebtInstrumentAxis_019dd484-1f4c-7a78-ad72-6d6a2fa4ba20" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019dd484-1f4c-7a78-ad72-6d6a2fa4ba20_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019dd484-1f4c-7a78-ad72-6d6a2fa4ba20" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019dd484-1f4c-7a78-ad72-6d6a2fa4ba20_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019dd484-1f4c-7a93-bf70-5518e3660712" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019dd484-1f4c-7a78-ad72-6d6a2fa4ba20" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019dd484-1f4c-7a93-bf70-5518e3660712" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_A8.50SeniorSecuredNotesDueApril2031Member_019dd484-1f4c-7c11-b6af-b2826616b21e" xlink:href="mnk-20260331.xsd#mnk_A8.50SeniorSecuredNotesDueApril2031Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019dd484-1f4c-7a93-bf70-5518e3660712" xlink:to="loc_mnk_A8.50SeniorSecuredNotesDueApril2031Member_019dd484-1f4c-7c11-b6af-b2826616b21e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RealizedInvestmentGainsLosses_019dd482-a210-7a2f-882e-a219fd359aaf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RealizedInvestmentGainsLosses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019dab17-c7e6-7d00-a0db-a882c900ebe2" xlink:to="loc_us-gaap_RealizedInvestmentGainsLosses_019dd482-a210-7a2f-882e-a219fd359aaf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrealizedGainLossOnInvestments_019dab17-c7e6-7e06-862f-7c81eee97d91" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrealizedGainLossOnInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019dab17-c7e6-7d00-a0db-a882c900ebe2" xlink:to="loc_us-gaap_UnrealizedGainLossOnInvestments_019dab17-c7e6-7e06-862f-7c81eee97d91" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedCash_019dab18-29a1-7e39-b131-a0cc901efe8a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedCash"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019dab17-c7e6-7d00-a0db-a882c900ebe2" xlink:to="loc_us-gaap_RestrictedCash_019dab18-29a1-7e39-b131-a0cc901efe8a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SelfInsuranceReserve_019dab18-29a1-7b77-9321-2e2a426aa751" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SelfInsuranceReserve"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019dab17-c7e6-7d00-a0db-a882c900ebe2" xlink:to="loc_us-gaap_SelfInsuranceReserve_019dab18-29a1-7b77-9321-2e2a426aa751" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EscrowDeposit_019dab18-29a1-7ea4-8bee-850275b3e32f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EscrowDeposit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019dab17-c7e6-7d00-a0db-a882c900ebe2" xlink:to="loc_us-gaap_EscrowDeposit_019dab18-29a1-7ea4-8bee-850275b3e32f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_BankGuaranteesLettersOfCreditSuretyBondsAndCollateral_019dfd5e-dc8e-78c2-ac8a-9f0ecf47928b" xlink:href="mnk-20260331.xsd#mnk_BankGuaranteesLettersOfCreditSuretyBondsAndCollateral"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019dab17-c7e6-7d00-a0db-a882c900ebe2" xlink:to="loc_mnk_BankGuaranteesLettersOfCreditSuretyBondsAndCollateral_019dfd5e-dc8e-78c2-ac8a-9f0ecf47928b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019dd484-1f4c-7c40-9852-72d55c13b32b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019dab17-c7e6-7d00-a0db-a882c900ebe2" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019dd484-1f4c-7c40-9852-72d55c13b32b" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofReconciliationofChangesinFairValueofContingentLiabilitiesDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#FinancialInstrumentsandFairValueMeasurementsScheduleofReconciliationofChangesinFairValueofContingentLiabilitiesDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofReconciliationofChangesinFairValueofContingentLiabilitiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019cd357-73c3-7c49-8488-06acf61fb0f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019cd357-73c3-7e50-80ce-dbbfd717ed56" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019cd357-73c3-7c49-8488-06acf61fb0f7" xlink:to="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019cd357-73c3-7e50-80ce-dbbfd717ed56" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cd357-73c3-7ffd-9928-083b3b3fc058" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019cd357-73c3-7e50-80ce-dbbfd717ed56" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cd357-73c3-7ffd-9928-083b3b3fc058" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-7ffd-9928-083b3b3fc058_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cd357-73c3-7ffd-9928-083b3b3fc058" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-7ffd-9928-083b3b3fc058_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-7491-ac29-1f0a36022eaf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cd357-73c3-7ffd-9928-083b3b3fc058" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-7491-ac29-1f0a36022eaf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019cd357-73c3-78d7-ace0-5a57d2d4d7cc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-7491-ac29-1f0a36022eaf" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019cd357-73c3-78d7-ace0-5a57d2d4d7cc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019cd357-73c3-724d-bb2a-8d228871463e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019cd357-73c3-7e50-80ce-dbbfd717ed56" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019cd357-73c3-724d-bb2a-8d228871463e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019cd357-73c3-724d-bb2a-8d228871463e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019cd357-73c3-724d-bb2a-8d228871463e" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019cd357-73c3-724d-bb2a-8d228871463e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019cd357-73c3-72a5-b95d-0c33f018f41c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019cd357-73c3-724d-bb2a-8d228871463e" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019cd357-73c3-72a5-b95d-0c33f018f41c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_TerlivazCVRMember_019cd357-73c3-7c74-8cea-55f5990831aa" xlink:href="mnk-20260331.xsd#mnk_TerlivazCVRMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019cd357-73c3-72a5-b95d-0c33f018f41c" xlink:to="loc_mnk_TerlivazCVRMember_019cd357-73c3-7c74-8cea-55f5990831aa" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_AuxiliumMember_019cd357-73c3-7e3b-8405-bc4377d3b6a0" xlink:href="mnk-20260331.xsd#mnk_AuxiliumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019cd357-73c3-72a5-b95d-0c33f018f41c" xlink:to="loc_mnk_AuxiliumMember_019cd357-73c3-7e3b-8405-bc4377d3b6a0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_019dff96-af79-7e61-952d-b4ebec813c5b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationTable_019cd357-73c3-7e50-80ce-dbbfd717ed56" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_019dff96-af79-7e61-952d-b4ebec813c5b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_019dff96-af79-7e61-952d-b4ebec813c5b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_019dff96-af79-7e61-952d-b4ebec813c5b" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_019dff96-af79-7e61-952d-b4ebec813c5b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_019dff96-af79-726d-97d0-547284628f85" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_019dff96-af79-7e61-952d-b4ebec813c5b" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_019dff96-af79-726d-97d0-547284628f85" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCurrentLiabilitiesMember_019dff97-53e3-7c5e-9a63-37811b4166b8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCurrentLiabilitiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_019dff96-af79-726d-97d0-547284628f85" xlink:to="loc_us-gaap_OtherCurrentLiabilitiesMember_019dff97-53e3-7c5e-9a63-37811b4166b8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_ContingentConsiderationRollForward_019cd357-73c3-7493-8b52-a4f845403ae9" xlink:href="mnk-20260331.xsd#mnk_ContingentConsiderationRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueLiabilitiesMeasuredOnRecurringBasisUnobservableInputReconciliationLineItems_019cd357-73c3-7c49-8488-06acf61fb0f7" xlink:to="loc_mnk_ContingentConsiderationRollForward_019cd357-73c3-7493-8b52-a4f845403ae9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_019dab06-2ba2-7b0e-b243-8b241e115936" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_ContingentConsiderationRollForward_019cd357-73c3-7493-8b52-a4f845403ae9" xlink:to="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_019dab06-2ba2-7b0e-b243-8b241e115936" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityFairValueAdjustments_019dab06-2ba2-79fe-8e56-2c4ef7cbb531" xlink:href="mnk-20260331.xsd#mnk_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityFairValueAdjustments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_mnk_ContingentConsiderationRollForward_019cd357-73c3-7493-8b52-a4f845403ae9" xlink:to="loc_mnk_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityFairValueAdjustments_019dab06-2ba2-79fe-8e56-2c4ef7cbb531" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue_019dab11-b4d4-738e-910d-434941fefede" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementWithUnobservableInputsReconciliationsRecurringBasisLiabilityValue"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofCarryingAmountandFairValueofLongtermDebtDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#FinancialInstrumentsandFairValueMeasurementsScheduleofCarryingAmountandFairValueofLongtermDebtDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/FinancialInstrumentsandFairValueMeasurementsScheduleofCarryingAmountandFairValueofLongtermDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019cd357-73c3-747c-896b-e0006785ca1a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019cd357-73c3-7996-9a01-c85380d237be" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByBalanceSheetGroupingTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019cd357-73c3-747c-896b-e0006785ca1a" xlink:to="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019cd357-73c3-7996-9a01-c85380d237be" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cd357-73c3-7ccb-b07e-5fbee26b4691" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019cd357-73c3-7996-9a01-c85380d237be" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cd357-73c3-7ccb-b07e-5fbee26b4691" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-7ccb-b07e-5fbee26b4691_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cd357-73c3-7ccb-b07e-5fbee26b4691" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-7ccb-b07e-5fbee26b4691_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-765a-90cd-ec207217d8ec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019cd357-73c3-7ccb-b07e-5fbee26b4691" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-765a-90cd-ec207217d8ec" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019cd357-73c3-7e52-a347-7cd5fae6c50e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-765a-90cd-ec207217d8ec" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019cd357-73c3-7e52-a347-7cd5fae6c50e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019cd357-73c3-788b-b0b7-816a2ebd6757" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019cd357-73c3-765a-90cd-ec207217d8ec" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019cd357-73c3-788b-b0b7-816a2ebd6757" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019cd357-73c3-7610-9dea-7975a7aae4c3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019cd357-73c3-7996-9a01-c85380d237be" xlink:to="loc_us-gaap_DebtInstrumentAxis_019cd357-73c3-7610-9dea-7975a7aae4c3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c3-7610-9dea-7975a7aae4c3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019cd357-73c3-7610-9dea-7975a7aae4c3" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c3-7610-9dea-7975a7aae4c3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c3-7fc9-ae7b-656b2a5492fd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019cd357-73c3-7610-9dea-7975a7aae4c3" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c3-7fc9-ae7b-656b2a5492fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_A8.50SeniorSecuredNotesDueApril2031Member_019cd357-73c3-71c7-b839-06a302151c6a" xlink:href="mnk-20260331.xsd#mnk_A8.50SeniorSecuredNotesDueApril2031Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c3-7fc9-ae7b-656b2a5492fd" xlink:to="loc_mnk_A8.50SeniorSecuredNotesDueApril2031Member_019cd357-73c3-71c7-b839-06a302151c6a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_TermLoanDueApril2031Member_019cd357-73c3-7b05-b87a-05392d6621ac" xlink:href="mnk-20260331.xsd#mnk_TermLoanDueApril2031Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019cd357-73c3-7fc9-ae7b-656b2a5492fd" xlink:to="loc_mnk_TermLoanDueApril2031Member_019cd357-73c3-7b05-b87a-05392d6621ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_019dab08-ca18-77ca-a36f-c797f457bec3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementBasisAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueByBalanceSheetGroupingTable_019cd357-73c3-7996-9a01-c85380d237be" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_019dab08-ca18-77ca-a36f-c797f457bec3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019dab08-ca18-77ca-a36f-c797f457bec3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019dab08-ca18-77ca-a36f-c797f457bec3" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019dab08-ca18-77ca-a36f-c797f457bec3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019dab13-5fc5-779d-a8ee-c58bc79c5cdb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019dab08-ca18-77ca-a36f-c797f457bec3" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019dab13-5fc5-779d-a8ee-c58bc79c5cdb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019dab08-ca18-7966-a4b8-92c9b6ef19ce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019dab13-5fc5-779d-a8ee-c58bc79c5cdb" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019dab08-ca18-7966-a4b8-92c9b6ef19ce" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_019dab08-ca18-78aa-b7ae-04fb8dbff6d3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019dab08-ca18-7966-a4b8-92c9b6ef19ce" xlink:to="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_019dab08-ca18-78aa-b7ae-04fb8dbff6d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019dab08-ca18-7c21-aca4-7edb5d179932" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019dab08-ca18-7966-a4b8-92c9b6ef19ce" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019dab08-ca18-7c21-aca4-7edb5d179932" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_019cd357-73c3-7189-88d4-8b8f111917c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019cd357-73c3-747c-896b-e0006785ca1a" xlink:to="loc_us-gaap_LongTermDebtFairValue_019cd357-73c3-7189-88d4-8b8f111917c6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019dfd60-0893-78ba-a456-d62be10bef45" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueBalanceSheetGroupingFinancialStatementCaptionsLineItems_019cd357-73c3-747c-896b-e0006785ca1a" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019dfd60-0893-78ba-a456-d62be10bef45" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/FinancialInstrumentsandFairValueMeasurementsSchedulesofConcentrationofRiskDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#FinancialInstrumentsandFairValueMeasurementsSchedulesofConcentrationofRiskDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/FinancialInstrumentsandFairValueMeasurementsSchedulesofConcentrationofRiskDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_019cd357-73c3-7327-be5c-6af5d51c5c5c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_019cd357-73c3-7d5d-ace9-8913246cfd36" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019cd357-73c3-7327-be5c-6af5d51c5c5c" xlink:to="loc_us-gaap_ConcentrationRiskTable_019cd357-73c3-7d5d-ace9-8913246cfd36" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019cd357-73c3-7ce8-b145-c2b24d0325c0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019cd357-73c3-7d5d-ace9-8913246cfd36" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019cd357-73c3-7ce8-b145-c2b24d0325c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019cd357-73c3-7ce8-b145-c2b24d0325c0_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019cd357-73c3-7ce8-b145-c2b24d0325c0" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019cd357-73c3-7ce8-b145-c2b24d0325c0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019cd357-73c3-7acb-be6f-2867df6eacef" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019cd357-73c3-7ce8-b145-c2b24d0325c0" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019cd357-73c3-7acb-be6f-2867df6eacef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_019dd4bc-26e9-7a11-a3cf-ccc9242e12f8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019cd357-73c3-7acb-be6f-2867df6eacef" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_019dd4bc-26e9-7a11-a3cf-ccc9242e12f8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019cd357-73c3-7559-991a-0527b6218bba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019cd357-73c3-7d5d-ace9-8913246cfd36" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019cd357-73c3-7559-991a-0527b6218bba" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019cd357-73c3-7559-991a-0527b6218bba_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019cd357-73c3-7559-991a-0527b6218bba" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019cd357-73c3-7559-991a-0527b6218bba_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019cd357-73c3-7613-93b0-70da392898c5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019cd357-73c3-7559-991a-0527b6218bba" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019cd357-73c3-7613-93b0-70da392898c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerMember_019cd357-73c3-7283-8618-2efff3992064" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019cd357-73c3-7613-93b0-70da392898c5" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerMember_019cd357-73c3-7283-8618-2efff3992064" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableMember_019cd357-73c3-7f8a-a9e0-571ca23ce61f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019cd357-73c3-7613-93b0-70da392898c5" xlink:to="loc_us-gaap_AccountsReceivableMember_019cd357-73c3-7f8a-a9e0-571ca23ce61f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_LegalEntityAxis_019cd357-73c3-77d8-af2e-445023d666b9" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_LegalEntityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_019cd357-73c3-7d5d-ace9-8913246cfd36" xlink:to="loc_dei_LegalEntityAxis_019cd357-73c3-77d8-af2e-445023d666b9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019cd357-73c3-77d8-af2e-445023d666b9_default" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_019cd357-73c3-77d8-af2e-445023d666b9" xlink:to="loc_dei_EntityDomain_019cd357-73c3-77d8-af2e-445023d666b9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_dei_EntityDomain_019cd357-73c3-7d92-9f19-413c9a4508a3" xlink:href="https://xbrl.sec.gov/dei/2025/dei-2025.xsd#dei_EntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_019cd357-73c3-77d8-af2e-445023d666b9" xlink:to="loc_dei_EntityDomain_019cd357-73c3-7d92-9f19-413c9a4508a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_FFFEnterprisesIncMember_019cd357-73c3-73da-b79e-ee4a492156b0" xlink:href="mnk-20260331.xsd#mnk_FFFEnterprisesIncMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019cd357-73c3-7d92-9f19-413c9a4508a3" xlink:to="loc_mnk_FFFEnterprisesIncMember_019cd357-73c3-73da-b79e-ee4a492156b0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_CencoraInc.Member_019cd357-73c3-7215-9d41-54a157c15f86" xlink:href="mnk-20260331.xsd#mnk_CencoraInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019cd357-73c3-7d92-9f19-413c9a4508a3" xlink:to="loc_mnk_CencoraInc.Member_019cd357-73c3-7215-9d41-54a157c15f86" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_CVSHealthCorporationMember_019dbf2c-1b40-7b64-a7cf-01c272664f88" xlink:href="mnk-20260331.xsd#mnk_CVSHealthCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_019cd357-73c3-7d92-9f19-413c9a4508a3" xlink:to="loc_mnk_CVSHealthCorporationMember_019dbf2c-1b40-7b64-a7cf-01c272664f88" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019cd357-73c3-706e-be6e-f8f4696d036d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_019cd357-73c3-7327-be5c-6af5d51c5c5c" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019cd357-73c3-706e-be6e-f8f4696d036d" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/SegmentDataScheduleofNetSalesfromExternalCustomersbyProductsDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#SegmentDataScheduleofNetSalesfromExternalCustomersbyProductsDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/SegmentDataScheduleofNetSalesfromExternalCustomersbyProductsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019cd357-746a-758d-9ba4-cc0db27fc0fc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019cd357-746a-7986-9c37-74170f91e89e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019cd357-746a-758d-9ba4-cc0db27fc0fc" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019cd357-746a-7986-9c37-74170f91e89e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019cd357-746a-757a-8901-9efb3aa66e94" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019cd357-746a-7986-9c37-74170f91e89e" xlink:to="loc_srt_ProductOrServiceAxis_019cd357-746a-757a-8901-9efb3aa66e94" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019cd357-746a-757a-8901-9efb3aa66e94_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019cd357-746a-757a-8901-9efb3aa66e94" xlink:to="loc_srt_ProductsAndServicesDomain_019cd357-746a-757a-8901-9efb3aa66e94_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019cd357-746a-7694-8c87-c0208f5573fd" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019cd357-746a-757a-8901-9efb3aa66e94" xlink:to="loc_srt_ProductsAndServicesDomain_019cd357-746a-7694-8c87-c0208f5573fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_ActharGelMember_019cd357-746a-7ddf-8950-0fb38ad2e870" xlink:href="mnk-20260331.xsd#mnk_ActharGelMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019cd357-746a-7694-8c87-c0208f5573fd" xlink:to="loc_mnk_ActharGelMember_019cd357-746a-7ddf-8950-0fb38ad2e870" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_XiaflexMember_019cd357-746a-70a9-8dc3-7f35a7e50131" xlink:href="mnk-20260331.xsd#mnk_XiaflexMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019cd357-746a-7694-8c87-c0208f5573fd" xlink:to="loc_mnk_XiaflexMember_019cd357-746a-70a9-8dc3-7f35a7e50131" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_InomaxMember_019cd357-746a-7c00-b2af-6943767c95f9" xlink:href="mnk-20260331.xsd#mnk_InomaxMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019cd357-746a-7694-8c87-c0208f5573fd" xlink:to="loc_mnk_InomaxMember_019cd357-746a-7c00-b2af-6943767c95f9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_AmitizaMember_019cd357-746a-78f5-929a-aa28db0ad628" xlink:href="mnk-20260331.xsd#mnk_AmitizaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019cd357-746a-7694-8c87-c0208f5573fd" xlink:to="loc_mnk_AmitizaMember_019cd357-746a-78f5-929a-aa28db0ad628" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_OtherProductsMember_019cd357-746a-7ece-b62a-95df0c47a68f" xlink:href="mnk-20260331.xsd#mnk_OtherProductsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019cd357-746a-7694-8c87-c0208f5573fd" xlink:to="loc_mnk_OtherProductsMember_019cd357-746a-7ece-b62a-95df0c47a68f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LicenseMember_019dab1f-6102-7a58-8957-9589e16522a7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LicenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019cd357-746a-7694-8c87-c0208f5573fd" xlink:to="loc_us-gaap_LicenseMember_019dab1f-6102-7a58-8957-9589e16522a7" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_019cd357-746a-77ba-b436-1eaac9c44972" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019cd357-746a-758d-9ba4-cc0db27fc0fc" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerIncludingAssessedTax_019cd357-746a-77ba-b436-1eaac9c44972" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.mallinckrodt.com/role/SharebasedCompensationDetails" xlink:type="simple" xlink:href="mnk-20260331.xsd#SharebasedCompensationDetails"/>
  <link:definitionLink xlink:role="http://www.mallinckrodt.com/role/SharebasedCompensationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019dabab-d244-7b91-a5e2-c8cd15e2160e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019dabab-d244-7956-9723-90fca2757c8e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019dabab-d244-7b91-a5e2-c8cd15e2160e" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019dabab-d244-7956-9723-90fca2757c8e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019dabab-d244-741a-b55f-61229b46345d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019dabab-d244-7956-9723-90fca2757c8e" xlink:to="loc_us-gaap_AwardTypeAxis_019dabab-d244-741a-b55f-61229b46345d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019dabab-d244-741a-b55f-61229b46345d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019dabab-d244-741a-b55f-61229b46345d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019dabab-d244-741a-b55f-61229b46345d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019dabab-d244-711c-bf77-83fc9580cc93" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019dabab-d244-741a-b55f-61229b46345d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019dabab-d244-711c-bf77-83fc9580cc93" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019dbf46-15b2-7d49-a3ac-14026d3d465c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019dabab-d244-711c-bf77-83fc9580cc93" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019dbf46-15b2-7d49-a3ac-14026d3d465c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019dfdd8-436a-77f5-86df-8e4e510ce75a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019dabab-d244-711c-bf77-83fc9580cc93" xlink:to="loc_us-gaap_PerformanceSharesMember_019dfdd8-436a-77f5-86df-8e4e510ce75a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_019dff8f-12de-7a1d-b2ab-48ef164257d2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019dabab-d244-7956-9723-90fca2757c8e" xlink:to="loc_us-gaap_VestingAxis_019dff8f-12de-7a1d-b2ab-48ef164257d2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_019dff8f-12de-7a1d-b2ab-48ef164257d2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_019dff8f-12de-7a1d-b2ab-48ef164257d2" xlink:to="loc_us-gaap_VestingDomain_019dff8f-12de-7a1d-b2ab-48ef164257d2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_019dff8f-12de-7149-88d9-12d334407c29" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_019dff8f-12de-7a1d-b2ab-48ef164257d2" xlink:to="loc_us-gaap_VestingDomain_019dff8f-12de-7149-88d9-12d334407c29" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_019dff8f-21f9-7576-99a8-cc8e32460847" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_019dff8f-12de-7149-88d9-12d334407c29" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_019dff8f-21f9-7576-99a8-cc8e32460847" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_019dff8f-422f-791b-b0d3-ed33674e7721" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_019dff8f-12de-7149-88d9-12d334407c29" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_019dff8f-422f-791b-b0d3-ed33674e7721" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019dabab-d244-7841-8e91-2ba9dc68b00c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019dabab-d244-7b91-a5e2-c8cd15e2160e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019dabab-d244-7841-8e91-2ba9dc68b00c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_mnk_ShareBasedCompensationArrangementsByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantedInPeriodFairValue_019e022e-233f-7232-9222-6c29a2592961" xlink:href="mnk-20260331.xsd#mnk_ShareBasedCompensationArrangementsByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantedInPeriodFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019dabab-d244-7b91-a5e2-c8cd15e2160e" xlink:to="loc_mnk_ShareBasedCompensationArrangementsByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantedInPeriodFairValue_019e022e-233f-7232-9222-6c29a2592961" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019dfdd8-436a-75f6-9de7-f9d3baebf66a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019dabab-d244-7b91-a5e2-c8cd15e2160e" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019dfdd8-436a-75f6-9de7-f9d3baebf66a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e054f-e8a7-7b28-9413-00685cfb1682" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019dabab-d244-7b91-a5e2-c8cd15e2160e" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019e054f-e8a7-7b28-9413-00685cfb1682" xlink:type="arc" order="3"/>
  </link:definitionLink>
</link:linkbase>
