Mouat had a very successful junior career, winning two Scottish junior championships in 2015 and 2016. He skipped the Scottish team at the 2015 World Junior Curling Championships, where he led his team of Duncan Menzies, Derrick Sloan and Angus Dowell to a bronze medal. The team had a 6-3 round robin record, which was good enough for third place. They lost to Sweden in the 3 vs. 4 playoff game, but won in a re-match against the Swedes in the bronze medal game. Mouat would again skip Scotland at the 2016 World Junior Curling Championships, this time with teammates Bobby Lammie, Gregor Cannon along with Dowell again. The team posted a 7-2 record after the round robin, in second place. In the playoffs, they would beat the United States in the 1 vs. 2 game, and beat them again in the final to claim the gold medal.