Apart of the U.S. development program, Mercier was named to the United States Team for the 2005 U18 World Championships. Far from the most skilled player, he was praised for his tenacity and tireless work as he recorded 1 assist in 6 games helping the U.S. capture Gold. In the gold medal game, a 5–1 victory over Canada, Mercier gained notoriety when he was assessed a match penalty and ejected in the third period following a dangerous hit along the boards against Colton Yellow Horn.