Mundell was returned to Parliament at the 2015 general election with a much reduced majority of 798 votes (1.5%), although the Conservative Party's share of the vote increased by 1.8%. He succeeded Liberal Democrat Alistair Carmichael as Secretary of State for Scotland. In the referendum on EU membership of June 2016, Mundell supported Britain remaining within the EU. Following it, he became a part-time member of the cabinet committee working on strategies for Brexit. He was the only Conservative MP to represent a Scottish constituency in the 56th parliament (2015–2017), but was joined by a further twelve in the 2017 snap general election. Following the election of Boris Johnson as Prime Minister, Mundell was sacked from his role and went to the backbenches for the first time since he was elected as an MP.