Playing major junior in the Ontario Hockey League (OHL), Mason spent three seasons with the London Knights and Kitchener Rangers.

Internationally, he won a gold medal with Team Canada at the 2008 World Junior Championships while earning tournament MVP and Best Goaltender honours.

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.

After defeating the Plymouth Whalers in the first round, Mason was pulled out of the line-up and underwent arthroscopic knee surgery on April 18, forcing him to miss the remainder of the OHL playoffs, as well as the 2008 Memorial Cup.

[3] Kitchener won the OHL championship without Mason, but lost 4–1 in the Memorial Cup final against the Spokane Chiefs of the Western Hockey League (WHL).

[4] Shortly after recovering and joining the Crunch's line-up, Mason was called up by the Blue Jackets on November 4, 2008, in the absence of injured starter Pascal Leclaire.

[6] Mason continued to start games upon Leclaire's return from injury and was named NHL Rookie of the Month for November after posting a 5–2–1 record, including three straight wins and two shutouts.

It was later revealed on January 30, 2009, by Blue Jackets General Manager Scott Howson that Mason had been suffering from mononucleosis for the previous three-to-four weeks.

The next three seasons were a disappointment for Mason, however, as he was unable to duplicate the play of his rookie year, and the Blue Jackets failed to make another playoff appearance.

[16] In the off-season, on June 25, 2013, Bryzgalov was bought out by the Flyers,[17] where the team subsequently signed goaltender Ray Emery, effectively promoting Mason to the starter's role.

He then led Team Canada to a gold medal at the 2008 IIHF World Junior Hockey Championships in Pardubice, Czech Republic.