Carlaw served as acting leader of the Scottish Conservatives while leader Ruth Davidson was on maternity leave from September 2018 until May 2019. Following her resignation in August 2019, he was appointed to serve a second term. In his role as acting leader, he supported Brexit and u-turned on criticisms of Prime Minister and Conservative leader Boris Johnson. He was the incumbent when Johnson called the 2019 general election, in which the party lost seven of their 13 seats from 2017 but held the remaining six.