Smith-Pelly and Alex Ovechkin scored, and Pheonix Copley made 24 saves for the Capitals (9-7-3), who had lost three of their previous four games.
"That was the most comfortable he has looked," Washington coach Todd Reirden said of Copley, who has been filling in for Braden Holtby (upper-body injury). "His rebound control was better and he was better playing the puck outside the crease."
Copley made 13 saves in the first period, including against a wide-open Gabriel Landeskog in the slot and a glove deflection against Nathan MacKinnon on a breakaway.
"He's been outstanding for us," Tom Wilson said. "Every time he's gone in net he's given us a chance to win, and more. That's all you can ask for from your goalie, and we try and go out there and pick up the rest of it. Love seeing that from a guy in his position."
Carl Soderberg scored for the Avalanche (9-6-4), who are 2-0-1 in their past three games. Grubauer, who was acquired in a trade from the Capitals on June 22, made 26 saves.
"Puck drops, it's all business. It doesn't matter who's on the ice," said Grubauer, who hadn't played since a 7-6 overtime loss to the Vancouver Canucks on Nov. 2. "It was nice to get a game in two weeks, or whatever, felt like it was the offseason. It was good, next game is coming up soon.
"Warmups, looking down, it was a little weird. It felt like a scrimmage in the preseason a little bit, looking down and seeing some familiar faces."