Mason's goals-against average of 2.39 ranks a mere 19th in the NHL this season, but he was solid against the Canadiens while helping Philadelphia to its 10th consecutive home win.
The Flyers have three straight victories overall and have captured seven out of eight.
Sean Couturier and Zac Rinaldo scored in the opening period for the Flyers and they took a 3-0 advantage midway through the second on a goal by Michael Raffl.
Andrej Meszaros had an assist on all three goals, the first time in his career he has produced that many assists in one game.
Montreal's only goal came from Tomas Plekanec with 2:23 left in the second period.
Peter Budaj stopped 24 shots in the loss. Montreal has dropped seven regular-season meetings in a row to Philadelphia.