Halak made his second start in four games, improving to 6-4.
"He was the reason we won the game tonight, for sure," Cammalleri said. "He was solid from the start. We needed him right away."
The Slovak goalie came up with a number of big stops in the first as Ottawa held a 13-5 edge in shots in the opening period.
"They came out hard and they were shooting," Halak said. "That's when I got in the game."
The Canadiens went 2-for-3 with the man advantage and successfully killed each of the Senators' seven power-play opportunities.
"I thought that everybody that played on specialty teams did an outstanding job," Canadiens coach Jacques Martin said.
Canadiens right winger Ryan White played four shifts for a total of 3:13 in the first before he was removed from the game because he was ineligible. Montreal recalled White from Hamilton of the AHL earlier in the day but neglected to inform the NHL about the roster move.
"I was informed that he was ineligible to play so we just continued with 11 forwards," Martin said.
White was called up because defenseman Paul Mara was unable to make the trip after he left Monday's game because of an upper body injury.
Montreal lost another defenseman in the first when Jaroslav Spacek left the game. He suffered a lower body injury and didn't return.
Cammalleri scored for a third straight game late in the first for Montreal, which beat Philadelphia 3-1 on Monday despite being held to 13 shots.
Mike Fisher scored his 13th goal early in the second for Ottawa, which had a five-game, home-winning streak snapped despite a 46-27 shots advantage.
"We got a lot of pucks at the net but just didn't do a good enough job of getting pucks in the net," Senators center Jason Spezza said.
Brian Elliot stopped 23 shots in his ninth straight start for the Senators, who earned a 4-3 shootout win at Anaheim on Sunday to wrap up a 1-3-1 trip.
Limited to one shot in the first period on Monday, the Canadiens struggled to get anything through to Elliott before Plekanec teamed up with Cammalleri on a 2-on-1 late in the period.
Checked by Ottawa defenseman Anton Volchenkov, Plekanec fed a pass to Cammalleri, who fired the puck into the left side of the net for his fifth goal in three games 18:32 in.
Fisher tied it 1:46 into the second when he was left alone in the slot to put Milan Michalek's rebound past Halak.
Plekanec, who assisted on Cammalleri's goal, helped set up Bergeron's second power-play goal in two games midway through the second period as Montreal regained a 2-1 lead.
The Czech center got his third point on Kostitsyn's fourth goal during a power play with 2:18 remaining in the game. Gomez scored his fourth goal into an empty net with 1:05 left.
- The Senators, 6-0-1 at home in November, began a stretch in which they will play 10 of 15 in Ottawa.
- The Canadiens have allowed just one goal in each of their last three wins after giving up a total of 15 during a 0-3-1 skid that preceded their current streak.