Jake Gardiner and Nikita Zaitsev scored, and Curtis McElhinney made 17 saves for the Maple Leafs (15-9-1), who are 1-2-1 in their past four games.
"That was a game that was very winnable for us," McElhinney said. "As a goalie you want to come up big and make a key save at a key moment and we just didn't get it tonight. So it's tough."
Ovechkin put the Capitals up 1-0 at 12:43 of the first period. He carried the puck through the neutral zone and got around Maple Leafs forward Leo Komarov, then skated into the zone, and, using defenseman Morgan Rielly as a screen, put a wrist shot from the top of the right circle past McElhinney's blocker.
"I thought he was 19 again," Capitals coach Barry Trotz said. "Hopefully he bumped his head and he can stay 19, I'll just keep telling him he's 19 or something. He was fantastic; when Alex is skating, he's a big, powerful man, and that shot, those things explode off his stick."
Capitals forward T.J Oshie, who had three assists, said he could tell early on that Ovechkin was going to have an impact.