Mason, in his first year with the Thrashers, was appearing in his 31st game of the campaign. He has 21 shutouts in a nine-year career that has included six seasons with Nashville and two with St. Louis.
The only goal came from Nik Antropov with 9:39 to go in the third period. It was his 14th of the season.
Philadelphia suffered its first shutout since March 6 and saw its lead atop the Eastern Conference reduced to a single point over Washington. The Flyers, however, have a game in hand against the Capitals.
Sergei Bobrovsky suffered the loss while stopping 21 shots.