Thornton was named to Canada's national under-20 team for the 1997 World Junior Championships in Switzerland. At 17 years old, he recorded four points in seven games, helping Canada to a gold medal. Two years later, he made his debut with the Canadian men's team at the 2001 World Championships in Germany. Thornton collected a goal and an assist over six games, as Canada was eliminated in the quarter-finals by the United States.