Steven Stamkos scored, and
Edward Pasquale
made 41 saves for Tampa Bay (60-16-4), which has clinched the Presidents' Trophy and is the third team in NHL history to win 60 games.
"It was a game where one team's season was on the line, and the other team's wasn't," Stamkos said.
The Lightning need to win their two remaining games, each on the road, to tie the 1995-96 Detroit Red Wings for most victories in a season in NHL history (62). They have lost three of their past five games.
"It's more important to get our game going and prepare for next week, but you know if we do that, we have a good chance to win both games," Tampa Bay defenseman Anton Stralman said.
Lehkonen batted the puck out of midair after Victor Mete's shot from the point was deflected, then reached around Pasquale to score on Montreal's 44th shot on goal.
Lightning coach Jon Cooper challenged for goalie interference by Lehkonen on his goal, but it was upheld by video review.
"When the bounces aren't going our way, you've just got to stick with it," Lehkonen said. "You've got to help your team any way you can. That's pretty much what I was trying to focus on. There's a lot of games; sometimes the bounces are there, sometimes they're not."
Domi scored on a breakaway on the 45th shot for the Canadiens.
"That was a heck of a play by [Lehkonen]," Domi said. "… He just flipped it over to me, and I got lucky and shot."
Stamkos gave Tampa Bay a 1-0 lead at 2:28 of the first period when his pass from the left corner went into the net off Domi's leg.