Mathurin played for Canada at the 2021 FIBA Under-19 World Cup in Riga and Daugavpils, Latvia after being cut from the Olympic team. On July 4, 2021, he scored a team-high 30 points, shooting 11-of-15 from the field and 6-of-9 from three point range, in a 100–75 group stage win over Japan. One week later, Mathurin scored 31 points to lead Canada to a 101–92 victory over Serbia in the third-place game and win the bronze medal. He averaged 16.1 points and four rebounds per game in the tournament.