Brendan Gallagher tied the game with 2:37 left in the third period and Max Pacioretty also scored for the Canadiens (22-9-5). Pacioretty's goal was his 10th in 12 games; Gallagher ended a 13-game goal drought.
"Everyone knows what I've been going through lately," Gallagher said, "and hopefully I don't have to wait that long for the next one."
Al Montoya, who spent the previous two seasons with the Panthers, made 31 saves for Montreal. He won for the first time since Oct. 26 after going 0-4-1 in his previous five decisions.
"It's emotional," Montoya said. "I've got a lot of buddies on that side. I put in a lot of hard work and time in with those guys and they gave me a great opportunity, and now I'm ready to seize it with another team and take it to another level."
Jason Demers and Vincent Trocheck scored for the Panthers (15-14-8), who have lost 10 of their past 13 games. Demers tied his career high with his seventh goal of the season. Reimer made 38 saves.
"We've just got to learn how to close out games as a team," Trocheck said. "We're sitting back too much once we get leads. We play our game up until we get a lead and then we get back on our heels. Any time (you do that), any good team is going to take it to you and get scoring opportunities, and that's what happened."