McAmmond was put on a line with Spezza and Heatley to provide defensive support.
"To play with those guys, I just try to do my part and get those guys the puck," he said. "Spezz had it and he made a great pass and I just tried to find that spot. That was a big game for us."
McAmmond took a pass from Spezza and beat Atlanta goaltender Kari Lehtonen to give the Senators their first win in three games between the teams this season.
Mike Comrie scored midway through the third to tie the game.
"It was a good hurdle for us to get over tonight, by showing that we could beat a physical, hard-hitting team," said Spezza, who was often on the receiving end of some extra attention from hulking Thrashers defenseman Andy Sutton.
"It was a pretty intense game," Spezza said. "It's playoff-type hockey."
Spezza and Daniel Alfredsson finished with two assists and Martin Gerber made 28 saves for Ottawa, which gave coach Bryan Murray career win No. 599. Only four coaches in NHL history have reached 600 victories.
Marian Hossa had two assists against his former team and Lehtonen finished with 25 saves as Atlanta lost its fourth straight game.
"It's frustrating," said Hossa, who has eight points, all assists, in three games against the Senators this year. "I think we played a solid game on the road and just the last 10 minutes.
Thrashers left wing Brad Larsen left the game in the second period and didn't return because of a lower-body injury. His status was not immediately known.
The Thrashers came in 1-5-1 in their past seven games.
Gerber, making his second straight start with Ray Emery serving the second of a three-game suspension, was fresh off a 28-save shutout of the Florida Panthers on Wednesday.
McCarthy beat him through a screen with a shot from the point on the power play 11 minutes into the game to put the Thrashers ahead.
Schaefer scored on a rebound with 32 seconds remaining in the period to even the score and Lehtonen had to be sharp to start the second, making a flurry of saves after Schaefer, Fisher and Alfredsson pinned Atlanta in its own end.
That allowed the Thrashers to retake the lead when Metropolit put a shot from close range past Gerber's shoulder.
Atlanta was on the power play less than three minutes later when Lehtonen gave up a big rebound on a shot from Antoine Vermette, allowing Kelly to score on the follow-up chance.
With Chris Phillips off for hooking, Kozlov snapped a shot between Gerber's pads from a bad angle to make it 3-2 after two periods.
Comrie scored on a rebound midway through the third to set the stage for McAmmond's go-ahead goal and Fisher capped things off with Lehtonen pulled in favor of the extra attacker.
"They got that rebound goal (Comrie's) that got the ball rolling and we couldn't stop it," Hossa said.
- The Senators scratched tough guy Brian McGrattan for the seventh straight game.
- Emery is eligible to return Thursday versus Buffalo
- Thrashers center Steve Rucchin missed his fourth straight game with an upper-body injury.
- Winger Denis Hamel faced the Senators for the first time since being claimed off waivers from Ottawa a week earlier
- Atlanta wraps up its road trip Tuesday at Carolina.