McDonald scored unassisted as he beat Carey Price with a wrist shot into the top left corner after St. Louis blew a 3-1 lead in the third period.
Scott Gomez tied it for the Canadiens with only 32.9 seconds remaining in regulation. Benoit Pouliot scored his second goal of the game midway through the third as Montreal rallied to earn a point after falling behind 2-0 just 3:35 in.
With Price pulled for an extra attacker, Gomez netted the tying goal when he deflected Jaroslav Spacek's drive from the point past Ty Conklin. A video review upheld referee Bill McCreary's ruling that Gomez didn't touch the puck with a high stick.
David Perron and Paul Kariya scored 38 seconds apart in the first and Alex Steen added St. Louis' third goal late in the opening period to give the Blues the 3-1 lead.
Perron scored during a power play 2:57 in and Kariya made it 2-0 when he beat Price from behind the net at 3:35.
Steen, who assisted on Perron's tally, scored at 19:47 as T.J. Oshie got his second straight assist.
Conklin stopped 35 shots for the Blues, who have won five of six. St. Louis improved to 5-3-1 under new coach Davis Payne, who took over when Andy Murray was fired on Jan. 2.
Pouliot brought Montreal within one twice, including his 10th of the season 9:11 into the third.
The Canadiens have lost three in a row (0-2-1).
Price, who made 24 saves, had an eventful opening period that included a spectacular collision with St. Louis forward Cam Janssen along the boards at 7:30.
Perron opened the scoring when he deflected Steen's point shot up and over Price's left pad.
Then, with Price on his knees and turned toward the net as he searched for the loose puck, Kariya jumped up to play it. He fired a shot from behind the goal line that banked in off Price's pad.
The sellout crowd came to life after Janssen sent Price flying backward to the ice when he drove along the right boards and collided with the goalie, who had ventured way out to play a loose puck.
Despite Montreal defenseman Roman Hamrlik's intervention, Price was looking for a fight. He tried to throw punches at Janssen, who was sent off for roughing with Hamrlik at 7:30.
Conklin was also penalized for crossing the center line during the melee, and Pouliot scored 8 seconds into the power play.
Steen restored the Blues' two-goal lead when he scored off a rebound of Jay McClement's shot on a 2-on-1.
Montreal made it 3-2 after a gaffe by St. Louis defenseman Barret Jackman in his zone. Gomez stripped the puck from Jackman and fed a pass to Pouliot, who fired in a one-timer for his eighth goal in 11 games.
Notes
McDonald ended a five-game goal drought. … Pouliot has eight goals in 13 games with Montreal since he was acquired from Minnesota on Nov. 23. He had two goals in 14 games with the Wild. … RW Mathieu Darche, recalled from Hamilton of the AHL on Tuesday, made his Montreal debut on a line with Tomas Plekanec and Mike Cammalleri. … The Canadiens dressed eight defensemen as Marc-Andre Bergeron and Ryan O'Byrne played between C Glen Metropolit on Montreal's fourth line.