Thormeyer began swimming at the age of 10 for the Markham Aquatic Club in Markham, Ontario but moved across the country to Delta, British Columbia shortly after he started. He continued to swim competitively under the Winskill Dolphins Swim Club through to his high school graduation. 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 the Canada's 2013 FINA World Junior Swimming Championships team 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. It wasn't until 2015 when Thormeyer made his first senior international appearance at the 2015 Pan American Games where he won a silver as a part of the 4x100 m freestyle relay. Later that summer at the 2015 FINA World Junior Swimming Championships, Thormeyer went along to along to win gold while also setting a new world junior record with Penny Oleksiak, Taylor Ruck, and Javier Acevedo on Canada's mixed 4 × 100 m freestyle relay. After winning a gold and two bronzes at the Canadian Olympic Trials in 2016, he was named to Canada's Olympic team for the 2016 Summer Olympics. Thormeyer swam the third leg of the men's 4 × 100 m freestyle relay which qualified for the final in fifth place, but ultimately placed seventh overall in the final. Later that year in December, Thormeyer won a bronze medal as part of Canada's 4x50m mixed freestyle event alongside Sandrine Mainville, Michelle Toro, and Yuri Kisil at the World Short Course Championships in Windsor, Ontario. Thormeyer is also a part of the University of British Columbia varsity swim team. Winning multiple individual events and contributing as a key swimmer for the relays earned him the title of both U SportsRookie of the Year and UBC Thunderbirds Rookie of the Year in his first year on the collegiate stage. He has proved himself to be a vital component to the UBC Thunderbird varsity swim team, contributing to multiple Canada West Championship titles and U Sport Championship national titles. In April 2017, Thormeyer was named to Canada's 2017 World Aquatics Championships team in Budapest, Hungary, where he contributed to a bronze medal for Canada in the 4 × 100 m mixed freestyle relay. He also was named to the Canada's 2017 Summer Universiade team and competed at the games that summer in TaiPei City, Taiwan.
In September 2017, Thormeyer was named to Canada's 2018 Commonwealth Games team. Thormeyer won a bronze medal in the 100 metre backstroke event. Competing backstroke for the first time on the international senior stage, he qualified for the final in second place with a significant best time. In the final however, he was edged out of silver by a couple tenths, taking the bronze.
