Mouat won his second WCT event at the 2017 Stu Sells Oakville Tankard with teammates Grant Hardie, Bobby Lammie and Hammy McMillan Jr.. It was the beginning of a very successful 2017–18 curling season. The team stayed in Oakville and won the Biosteel Oakville Fall Classic the following week, defeating the same team, Kim Chang-min of Korea in the final. Later on in the season, the Mouat rink would once again beat Kim in another final, this time at the 2017 Boost National. Team Mouat won the event, capturing his first career Grand Slam title, and became the youngest skip and only the second ever non-Canadian skip to ever win a men's Grand Slam event. Later in the season, he won the Dumfries Challenger Series and Aberdeen International Curling Championship back home in Scotland. He also won his first Scottish Men's Curling Championship in 2018. This qualified his team to represent Scotland at the 2018 World Men's Curling Championship. His team finished the round robin with an 11–1 in a tie for first, but lost in the semifinal to Canada but went on to beat Kim Chang-min's South Korean team in the bronze medal game. Mouat played in three other slams that season, failing to qualify at the Canadian Open, making it to the quarterfinals of the Players' Championship and the semifinals at the Champions Cup.