In 2004–05, while attending Abbey Park High School, he led the Grimsby Peach Kings club to the OHA Jr.C. title in 2004–05 and in 2005–06 was assigned to the Petrolia Jets Jr.B. club when his playing time was limited with London. Despite this, Mason was selected by the Columbus Blue Jackets in the third round, 69th overall, in the 2006 NHL Entry Draft. He started the 2007–08 season with the Blue Jackets, but was returned to London after being a healthy scratch for Columbus' first two games of the season. While competing for Team Canada at the 2008 World Junior Championships, he was traded by the Knights to the Kitchener Rangers on January 4, 2008, hours before the semi-final game.