Mark Philip Sedwill, Baron Sedwill, is a British diplomat and senior civil servant who served as Cabinet Secretary and Head of the Home Civil Service to Prime Ministers Theresa May and Boris Johnson from 2018 to 2020. He also served as the United Kingdom National Security Adviser from 2017 to 2020. He was previously the United Kingdom's Ambassador to Afghanistan from 2009 to 2010 and the NATO Senior Civilian Representative in Afghanistan in 2010. He was the Permanent Under-Secretary of State at the Home Office from February 2013 to April 2017.

The Cabinet Secretary is the most senior civil servant in the United Kingdom and is based in the Cabinet Office. The person in this role acts as the senior policy adviser to the Prime Minister and Cabinet, and as the Secretary to the Cabinet is responsible to all ministers for the efficient running of government. The role is currently occupied by Simon Case.