Montreal got 29 saves by Carey Price to beat the Senators at Bell Centre in the back end of a home-and-home series.
Tomas Plekanec's goal at 18:06 of the middle period broke a 1-1 tie, and Max Pacioretty added some insurance by beating Craig Anderson midway through the final period.
"We had a pretty good lineup tonight, and it's nice to go out with a win," Montreal captain Brian Gionta said one night after a lineup made up mostly of young players lost 5-2 at Canadian Tire Centre. "At the same time, there are still things that need to be improved."
The Canadiens open Tuesday night at home against the Toronto Maple Leafs. Ottawa completes its preseason Sunday against the New York Islanders.
Ottawa's Cory Conacher tapped home a loose puck 5:09 into the game to open the scoring. Alex Galchenyuk tied it late in the first by finishing off a three-way passing play with Lars Eller and Daniel Briere. Plekanec put the Canadiens ahead to stay when he roofed a shot from the left circle.