Julian Carranza gave Philadelphia a 21st minute lead as he drew a foul that led to a penalty that he was able to convert, scoring for the third time in the last four matches.
Montreal would grab a point as second-half substitute Kei Kamara scored just before the hour mark to keep the Canadian side sixth in the East standings.
Story of the match
In the fourth minute, Philadelphia had their first sight of goal when Kai Wagner sent a long volley to Mikael Uhre and after dribbling into the box, the Dane found Carranza, his layoff pass to Daniel Gazdag intercepted.
Eight minutes later, Gazdag found some space, pulled a pair of defenders with him and dropped a pass to Leon Flach, who sent a difficult cross through the red zone searching for Uhre.
In the 19th minute, the Union broke through. Uhre provided the pressure near the halfway line with Wagner and forced a turnover. He quickly got Gazdag into the attack and the Hungarian tried to play Carranza in on goal.
Kamal Miller reacted late, dragging down the Argentinean, referee pointing to the PK spot. Carranza stepped up and calmly finished past Sebastian Breza.
Montreal turned up the pressure in search of an equalizer and on the half-hour mark, Ismael Kone let one fly from 25 yards out that was easily blocked by Philadelphia goalkeeper Andre Blake.
Kamara was introduced at halftime and the move paid off as he finished off a cross from Romell Quioto that leveled the score in the 59th minute.
The Union had a half-chance two minutes after the goal when Joel Waterman was booked for a hard tackle on Uhre and Gazdag's free-kick sailed over the crossbar.
In the 81st minute, Wagner connected with Uhre in the final third but a timely tackle from Victor Wanyama saw the play come to an end as the match would finish as it started.
Man of the match: Kei Kamara
He proved to be an inspired substitution after scoring the equalizer. The former Sierra Leone international has now scored more goals against Philadelphia (eight) than any other player in league history, doing it with four different teams.