The Union had a chance to move into first place in the East, but Geoff Cameron's equalizer with minutes to play ruined their hopes.
Despite leading for most of the night, the Union watched the Houston Dynamo's Cameron score in the final minutes as the two teams played to a 1-1 draw at PPL Park. Philadelphia remains in second place behind the Columbus Crew.
Cameron, one of Houston's four All-Stars this season, now has four goals on the season after his 85th-minute strike got past goalkeeper Faryd Mondragon.
Philadelphia's Jack McInerney had the first goal of the game and his first of the season in the 15th minute. Sebastien Le Toux picked up the assist, his team-leading eighth of the year.