Stewart had considered a parliamentary career in the past but only decided to stand when, in the aftermath of the expenses scandal, David Cameron decided to "reopen the Conservative candidates' list to anybody who wants to apply". Stewart has said that his experience in Afghanistan made him a "Burkean conservative". Having never voted for the Conservatives before (though, against his will, his parents cast his proxy vote for them in the 2001 general election, when he was abroad), he joined the party in summer 2009. Stewart tried for selection for the Bracknell constituency in the 2010 general election, but the place went to Phillip Lee. Stewart was then shortlisted for the Penrith and The Border constituency and, at an open caucus, selected as the candidate on 25 October 2009. He was returned as the MP for the constituency on 6 May 2010. At the 2015 general election, Stewart almost doubled his majority in Penrith and the Border from 11,241 to 19,894, the highest majority since the seat was created. In the 2017 general election, he received 60.4% of the vote.