In 2010, Stewart was elected to the House of Commons, and in 2014 was elected chair of the Defence Select Committee. He served under David Cameron and Theresa May as Minister for the Environment, Minister of State for International Development, Minister of State for Africa and Minister of State for Prisons. He ultimately joined the Cabinet as Secretary of State for International Development in the majority May government. After May resigned, Stewart stood as a candidate to be Leader of the Conservative Party and Prime Minister of the United Kingdom in the 2019 leadership contest. His campaign was defined by his unorthodox use of social media and opposition to a no-deal Brexit. Despite winning the backing of several senior cabinet ministers, he was eliminated from the race after a poor performance in the second leadership debate.