The Canadiens added four goals in the second as Francis Bouillon, Andrei Markov, Mikhail Grabovski and Tom Kostopoulos scored on Ray Emery, who replaced Martin Gerber after he had an awful opening period.
Carey Price stopped 31 shots for Montreal, which returned to the playoffs for just the fourth time in the past nine seasons.
The Canadiens, who were eliminated from playoff contention last season with a loss in Toronto in their final regular-season game, lead the Eastern Conference with 96 points with only five games remaining.
Montreal extended its Northeast Division lead to seven points over Ottawa, which has six games remaining, including Tuesday night in Buffalo.
Dany Heatley got his 38th and 39th goals for the Senators, who scored four times in the third period after falling behind 7-1 late in the second.
Antoine Vermette, Martin Lapointe and Cory Stillman also scored for Ottawa, which had won five of its six previous games against Montreal this season.
ISLANDERS 4, PENGUINS 1
In Uniondale, N.Y., Bill Guerin had a goal and assist in the decisive second period for the also-ran New York Islanders, who prevented Pittsburgh from clinching a playoff spot.
Pittsburgh holds a two-point lead over New Jersey in the race for the Atlantic Division crown but missed a chance to double its advantage heading into today's road meeting with the Devils.
Penguins captain Sidney Crosby sat out his sixth consecutive game because of a sprained right ankle, but he's close to return- ing -- perhaps today. Pittsburgh is 15-8-4 without him.
Richard Park, Sean Bergenheim and Trent Hunter also scored for the Islanders, and Wade Dubielewicz stopped 28 shots.