In May 2015, Brown was elected leader of the Ontario PC Party, and stepped down as a federal MP. He then won a provincial by-election on September 3, 2015, in Simcoe North and entered the Legislative Assembly of Ontario. Brown resigned as party leader in January 2018 after allegations of sexual misconduct, was expelled from the party caucus by interim leader Vic Fedeli on February 16, 2018, and left provincial politics after an aborted second run for the PC leadership Brown became the first permanent Ontario Conservative leader who did not lead the party into an election since George Frederick Marter.