MacKinnon's second of the night and 35th of the season came 2:52 later on his very next shift, burying a loose puck in front after Tyson Barrie's point shot.
That marker held as the game-winner, and MacKinnon is now tied for first in the NHL with 11 game-winning goals on the season--third most in franchise history behind only Sakic (12, Colorado, 2000-01) and Michel Goulet (16, Quebec, 1983-84).
"We were a little opportunistic tonight. We scored on the right chances, and we did some good things," said Avalanche head coach Jared Bednar. "I think [the Blues] were pushing real hard, they were coming at our D-core, they were coming at us in our own end. I thought Varly was outstanding again. We had the right guys [be] difference makers tonight. Our top guys were the top guys all around."
MacKinnon assisted on Mikko Rantanen's insurance goal into the empty net with 48 seconds remaining for his 85th point of the campaign, fourth most in the league. The helper also allowed him to become the first Avalanche player to reach 50 assists in a season since 2009-10 (Paul Stastny, 59).