Guillaume Latendresse scored two power-play goals to lead the Canadiens. Mike Komisarek, Radek Bonk, Mike Johnson and Alex Kovalev also scored for Montreal (9-4-3) and Cristobal Huet stopped 36 shots. Denis Hamel, Patrick Eaves and Antoine Vermette scored for Ottawa (6-10-1) and Martin Gerber faced 31 shots in his eighth loss of the season.
The Senators are now a dismal 2-6-0 on home ice and have yet to win a game this season when they fail to score first.
Goaltending continues to be Ottawa's biggest Achilles heel as Gerber struggled yet again.
Montreal opened the scoring at the 12-minute mark of the first as the Senators continued to struggle on the power play as they gave up their seventh shorthanded goal, a league worst. Tomas Plekanec passed to Komisarek who's shot just trickled through Gerber's pads and across the goal-line.
The Senators tied the game with less than two minutes remaining in the opening period as Denis Hamel tipped Andrej Meszaros' shot from the blue-line.
The Canadiens regained the lead late in the second period as Bonk, standing at the top of the crease, was able to knock down Alexander Perezhogin's wrist shot.
Montreal took control of the game early in the third as Latendresse scored two power-play goals within 31 seconds to help the Canadiens jump out to a 4-1 lead.
On the first, Latendresse caught Gerber out of position and on the second he beat him with a quick shot.
Ottawa managed to make the game interesting for the 20,051 on hand at Scotiabank Place scoring a pair of their own power play goals, but failed to tie the game.
Ottawa cut the lead in half as Eaves scored a power-play goal at the seven-minute mark and Vermette scored with just over six minutes remaining, but it meant nothing as Johnson beat Gerber on an ugly goal to seal the victory.
The Senators will have little time to regroup as they take on the Eastern Conference leading Buffalo Sabres Wednesday night.
Notes: Ottawa D Wade Redden (groin) is out of the lineup indefinitely, G Ray Emery (wrist, day-to-day). Montreal was without LW Chris Higgins (ankle, mid-December), D Mathieu Dandenault (hamstring tear, out indefinitely) and D Francis Bouillon (knee, out indefinitely).