Few goaltenders have been drafted into the NHL with less experience than Mason, who was selected by the Columbus Blue Jackets in the third round (No. 69) in the 2006 NHL Draft after having played just 12 games as a rookie with London of the Ontario Hockey League.

The Blue Jackets never considered the move to be much of a gamble, and it paid off for them when Mason won the Calder Trophy as the NHL's top rookie for the 2008-09 season. Mason also helped guide the Blue Jackets to their first Stanley Cup Playoff appearance that season, when he had 33 regular-season wins, a 2.29 goals-against average (second in the NHL), a .916 save percentage and a league-leading 10 shutouts in 61 games (all starts).

Mason also finished fourth in the voting for the Hart Trophy, awarded to the NHL's most valuable player, and second in balloting for the Vezina Trophy (best NHL goaltender) that season, when he was also named to the NHL All-Star Second Team and set Blue Jackets records for wins and shutouts in a season.

After four-plus seasons with Columbus, Mason was traded to the Philadelphia Flyers on April 3, 2013, for Michael Leighton and a third-round draft pick in 2015.

The Flyers Fan Club voted Mason the winner of the Gene Hart Memorial Award as the player who showed the most heart during the 2013-14 season, his first in Philadelphia. He went 33-18-7 with a 2.50 GAA and a .917 save percentage that season.

A native of Oakville, Ontario, Mason was named OHL Goaltender of the Year after he set a league record by winning 45 regular-season games for London in 2006-07. During the following season, Mason played in the World Junior Championship, where he was named the tournament's MVP and top goaltender after helping to lead Canada to a fourth straight gold medal.