The 36-year-old native of Livonia, Mich., beat Antero Niittymaki on a rebound at 9:36 of the third period to move to within two goals of Joe Mullen for the career lead among American players.
Modano joined Brendan Shanahan, Jaromir Jagr, Joe Sakic, Peter Bondra, Mats Sundin, Teemu Selanne and Mark Recchi as active 500-goal scorers.
With the game tied 1-1, Jussi Jokinen gave the Stars the lead at 9:02 of the third period on a wrist shot from the slot. Just 34 seconds later, Modano extended the lead to 3-1 with his 15th goal of the season and fourth in six games, knocking home the rebound of Jon Klemm's shot.
Scottie Upshall and Joni Pitkanen scored for the Flyers, 3-6-3 in their last 12.