 As a free agent Lehtonen gained NHL interest and on 4 May 2020, the Toronto Maple Leafs, signed Lehtonen to a one-year entry level contract for the 2020–21 season.Internationally, Lehtonen has represented Finland at multiple events, include the World Championships in 2017 and 2019, winning gold at the 2019 tournament and earning recognition on the World Championship All-Star Team.He also skated for Finland at the Olympic Games in 2018 and the 2014 World Junior Championships, where he captured gold.