Ryan O'Reilly added a goal, and Jean-Sebastien Giguere made 29 saves for the Avalanche, whose 12-1 start is the best in the team's 18 seasons in Colorado and matches the beginning of the 1994-95 Quebec Nordiques.
"We're 12-1, and everyone in this locker room feels like we're not satisfied," MacKinnon said. "That's the exciting part, that we can get better; 12-1 is great, but 13-1 is even better."
Landeskog, who had the game's first goal, fired a shot from the wing that bounced away from goalie Peter Budaj. MacKinnon backhanded the rebound into the net to put the Avalanche on top 2-1 at 8:01 of the third.
"It was a great shot from Landy and a great rebound from Nate," Avalanche coach Patrick Roy said.
MacKinnon said he happened to be in the right spot for the decisive shot.
"I was on the wall, and Landy fired it off his pad, and it came to my blade," MacKinnon said.
A little more than three minutes later, O'Reilly got a wraparound shot through Budaj, a former Avalanche player, for another goal.
"Playing against my ex-team, it was fun, it was a good atmosphere, especially with them being one of the best teams in the league," Budaj said. "It was a good experience for me. We battled hard. It's a tough result."
Montreal snapped Colorado's season-high streak of 22 consecutive penalty kills when Gallagher scored at 3:38 of the third period.
With Maxime Talbot in the box for slashing, P.K. Subban took a shot from the point. Giguere couldn't quite corral the puck, and Gallagher -- positioned at the top of the crease -- knocked the rebound in to tie it 1-1.
Landeskog had just started a shift when he got a lead pass from Marc-Andre Cliche. He built up steam as he pushed the puck down the ice before pulling up in the middle of the left circle and slapping a high shot that rocketed over Budaj's shoulder for a 1-0 lead 11:14 into the second.
"High is usually tough for goalies coming down the wing," Landeskog said. "I'm not sure if mine got tipped or not, but I'll take it."
The teams fought through a scoreless first period with both Budaj and Giguere making key saves.
Before the game, the Avalanche retired the No. 52 jersey of longtime defenseman Adam Foote in a ceremony that had the crowd roaring before the puck was dropped.
Foote, whose 19-year NHL career included two Stanley Cup titles with Colorado, was praised as the ultimate competitor and warrior by his former teammate and current Avalanche executive Joe Sakic.
Colorado LW Jamie McGinn, who has missed three games with a knee injury, has resumed practicing and is expected to return Wednesday against Nashville. ... Cody McLeod was back for Colorado after serving a five-game suspension for his hit on Detroit's Niklas Kronwall. ... Andrei Markov is one game away from his 700th played with the Canadiens. ... Colorado's Alex Tanguay sustained a knee injury in the second period and didn't return. ... The Canadiens lost both ends of a back-to-back set on the road. On Friday, Montreal was beaten 4-3 at Minnesota. ... The Avalanche won both ends of the first of 10 back-to-back sets this season.
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DENVER, CO - NOVEMBER 02: The jersey of retired player Adam Foote #52 of the Colorado Avalanche is hoisted to the...