Toronto suffered a demoralizing loss on Saturday by blowing a late lead but Sundin helped them leapfrog Boston in the standings. Monday's victory gives Toronto 42 points, one more than last place Boston in the Northeast Division.
Zdeno Chara scored for the Bruins, netting a rebound at 4:51 of the first as he was being checked to the ice. Boston had won four of its last six games.
The Maple Leafs are 18-17-6 at the halfway mark of their season.
Sundin tied it 1-1 at 8:09 of the first with a power-play goal from the side of the net that got past goalie Tim Thomas. He added his 18th of the season by skating up the ice and putting a hard slap shot past Thomas, giving Toronto a 4-1 lead at 2:00 of the third.
Sundin assisted on O'Neill's goal at 5:13 of the third.
Gil gave Toronto a 2-1 lead at 12:19 of the first by throwing the puck on net from the point.
Newbury made it 3-1 at 12:24 of the second with a weak shot from the point that beat Thomas. Chara and Toronto enforcer Wade Belak battled in front of the net and along the boards just before and after the goal.
Both received roughing penalties.
Toronto goalie Andrew Raycroft has struggled of late, but he looked good against his former team. He made 27 saves, including several impressive stops in the third.
Toronto's Darcy Tucker returned after missing two games with an injured left foot. Boston's Paul Mara checked Tucker in the boards in the first period. Tucker stayed down for a while, but stayed in the game.