Jonathan Toews scored his first goal of the season for Chicago (10-14-2), which had lost two of its past three.
"It's amazing," Toews, the Blackhawks captain, said. "I mean, so fitting he gets this win here in Montreal, too, with the greats that he's joined with 500 wins. Just the home of so many great goaltenders here in Montreal, so pretty cool for everyone in that locker room. I think we all feel pretty honored to be a part of that one."
Jake Allen made 23 saves for Montreal (6-19-3), which has lost five straight games (0-4-1) and is 4-10-1 at home.
"It must be special for [Fleury], especially doing it here," Canadiens coach Dominique Ducharme said. "And you know, 500 wins, you've got to tip your hat. That's a pretty nice career. We would have liked for him to get it, but not here, not against us."
Toews put the Blackhawks ahead 1-0 at 13:48 of the second period, deflecting a pass from Seth Jones near the right post on the power play.
The goal was his first since March 5, 2020, after he did not play last season because of what is being called chronic immune response syndrome.
"It was like three or four guys pulling the monkey off for me," Toews said. "It felt good. It was definitely weird to have a whole team probably jump on the ice and celebrate with me."