General election, 2015

At the 2015 general election, Brown was elected to Parliament as MP for Kilmarnock and Loudon with 55.7% of the vote and a majority of 13,638.^[7][8][9] He made his maiden speech on 22 June, in which he quoted the poem Is There for Honest Poverty by Robert Burns.^[10][11]

In 2016, Brown was one of 58 Scottish MPs who voted against the renewal of the UK's Trident nuclear programme. He claimed the programme had not served as a deterrent and that each job created through it cost the UK taxpayer £6.5 million.^[12][13]

Brown has revealed that Hansard reporters in Parliament often ask him to provide written 'translations'' of his questions to the Commons due to his thick Ayrshire accent being difficult to understand. Even so, he has said he would not alter his accent because his constituents “know me locally and know how I talk, they would actually question what was happening if my accent changed when I came down to Westminster”.^{[14][15][16][17]}

Brown was re-elected as MP for Kilmarnock and Loudoun at the snap 2017 general election with a decreased vote share of 42.3% and a decreased majority of 6,269 votes.^[18][19]

On 20 June 2017, Brown was appointed to the Frontbench Team of Ian Blackford as the SNP spokesperson on Transport, Infrastructure and Energy in the Palace of Westminster.^[20] In 2019 during a reshuffle, Brown was appointed to the role of SNP spokesperson on Energy and Climate Change by Iain Blackford.^[21]

Brown also held the position of spokesperson for the SNP in Westminster for Transport, a position held from 2017 until 2020. Brown currently sits on the Business, Energy and Industrial Strategy Committee within the House of Commons.^[22]

At the 2019 general election, Brown was again re-elected, with an increased vote share of 50.8% and an increased majority of 12,659.^[23][24]