Mason first competed for Team Canada at the under-20 level in the eight-game 2007 Super Series against Russia. He then led Team Canada to a gold medal at the 2008 IIHF World Junior Hockey Championships in Pardubice, Czech Republic. Prior to the semi-final game against Team USA, Mason was traded within the OHL from the London Knights to the Kitchener Rangers. After defeating Team Sweden 3–2 in overtime of the gold medal game, he was named as the player of the game, as he was in the semi-finals. Mason was undefeated in five games with a tournament-best .951 save percentage and 1.19 GAA. He was named to the tournament all-star team, received top goaltender honours, and was named tournament MVP.