It showed as the Leafs, who beat Ottawa 4-3 in a shootout Saturday, controlled the early going and led 1-0 after the first. But the Bruins' three-goal outburst in the second period, including a pair of power-play goals in a 94-second stretch, left the Leafs in their wake.
Toronto, which also gave up two power-play goals against the Senators, went 0 for 4 on its power play Sunday. The Bruins outshot the Leafs 40-32.
Carl Soderberg, Torey Krug, Kevan Miller, Jarome Iginla and Patrice Bergeron — with an empty-net goal coming out of the penalty box with 11 seconds remaining — scored for Boston, which came into the game under the spotlight thanks to Thornton's meltdown at TD Garden.
The Bruins tough guy is suspended pending a league hearing into a sucker-punch attack that sent Pens defenceman Brooks Orpik to hospital.
"I agree, he did cross the line," Boston coach Claude Julien said of Thornton prior to Sunday's game. "He got caught in the emotions. There's nobody that's proud of what happened (Saturday) night. Absolutely not. So he's going to suffer the consequences and so will we."
"Thorny did cross the line and some others did too," Julien added. "But sometimes you have to man up to those things and I think he did."
Defencemen Adam McQuaid and Johnny Boychuk were sidelined with earlier injuries. The Bruins' defensive woes were intensified Sunday when Dougie Hamilton exited after the first period after a collision with Leafs blue-liner Carl Gunnarsson.
It took the Leafs almost five minutes to get a shot on the Boston goal but they still showed jump and went ahead at 12:20 when Holland jammed a David Clarkson redirect of a Jake Gardiner shot into a gaping net for his third of the year.
Toronto, outshot in 26 of its first 30 games, outshot the Bruins 11-10 in the first period.
It took a while before shot No. 12 came, however. And Boston pulled even on the power play at 5:14 of the second period after Reilly Smith found Soderberg alone on the edge of the crease. Toronto's Carter Ashton was in the penalty box for delay of game.
For Smith, who grew up in nearby Mimico, it was a point in his first appearance at the Air Canada Centre.
Twenty seconds after the goal, the Leafs went a man-down again with Gunnarsson sent to the box. And Krug made the Leafs pay with his eighth goal of the season at 6:47, beating Bernier with the hulking Zdeno Chara blocking the goalie in front.
Ten of the Bruins' 15 power-play goals this season have come from their defencemen.
Boston outshot Toronto 8-0 in the second period before the home side finally put a shot on Johnson at 9:57. Defenceman Cody Franson had a good chance from in close with seven minutes remaining but the Bruins goalie made the save.
Smith hit the Toronto crossbar late in the period. Seconds later, Miller's wrist shot from the blue-line slipped through Bernier at 15:58. It was his first NHL goal.
Boston outshot Toronto 17-9 in the second period.
McClement closed the gap to 3-2 just 37 seconds into the third, winning a faceoff in the Bruins] end and then driving the Boston goal, slapping in his own rebound for his first of the year .
The Bruins had to kill off consecutive penalties to maintain their lead. The Leafs, meanwhile, need a huge save from Bernier on Jordan Caron to stay within one as the clock wound down.
But he was powerless to stop Iginla after Milan Lucic accelerated past Phaneuf and fed Iginla in the crease for his sixth goal at the 16:00 mark.
Miller left the game with 2:17 remaining after being hit in the boards by Phaneuf. And Boston closed out the game killing off the Bergeron penalty while the Leafs pulled their goalie for a sixth attacker.
The ugliness in the Bruins-Penguins game overshadowed the fact that the Sunday's contest marked the Bruins' first visit to Toronto since their playoff win last season.
The Bruins won the first matchup this season, beating the visiting Maple Leafs 3-1 on Nov. 9.
NOTES — Sunday's game was the 326th regular-season game meeting between the Bruins and Leafs in Toronto but just the third on a Sunday. ... The Bruins continue their four-game road trip in Canada against Calgary on Tuesday, Edmonton on Thursday and Vancouver on Saturday before returning home to host the Flames on Dec. 17.