The Flyers won their third straight and moved within a point of the New York Rangers for second place in the Metropolitan Division.
Mason did not allow a goal until Nathan MacKinnon put one past him with just 2:50 to play. That cut the Colorado deficit to 2-1, but Claude Giroux wrapped up the Philadelphia win by scoring into an empty net with 20 seconds remaining.
The Avalanche lost their second in a row.
Mark Streit and Michael Raffl scored for Philadelphia with Streit's goal coming on the power play 6:21 into the second period.
Semyon Varlamov gave up two goals on 34 shots for Colorado.