As an age-group swimmer, Thormeyer made a name for himself on the Canadian swimming scene. At 15 he competed at the 2013 Canada Summer Games for Team British Columbia, where he earned a bronze in the men's 100m backstroke. He also represented Canada multiple times on the junior international stage, being the youngest male on Canada's team at the 2013 World Junior Swimming Championships and winning five bronze medals at the 2014 Junior Pan Pacific Championships. These accomplishments earned him the Delta Sport's Hall of Fame 2014 Youth Athlete of the Year and the Sport BC Junior Male Athlete of the Year in 2016.