Chara, one of the NHL's top free agents last summer, scored a power-play goal on a blistering slap shot from the right point with 4:25 left to give the struggling Bruins just their second win in six games.
The Bruins also hired away former Ottawa assistant general manager Peter Chiarelli in May, naming him the team's GM after firing Mike O'Connell. Chiarelli later fired coach Mike Sullivan and replaced him with Dave Lewis.
Tim Thomas stopped 35 shots and P.J. Axelsson also scored for Boston.
Dany Heatley scored a second-period power-play goal for the Senators, who had a three-game winning streak snapped. Martin Gerber had 30 saves for the Senators.
After a scoreless first period, the Senators took advantage of a two-man advantage to grab the lead on Heatley's goal at 17:21 in the second. Heatley, positioned at the right side of the crease, banged in a cross-ice pass from Jason Spezza.
The Bruins tied it with 12:55 to play in the third when Axelsson stole the puck from defenceman Tom Preissing, broke in alone and fired a wrist shot into the top left corner.
The Senators had scored a club-record tying 21 goals in their previous three games.
Chara used his 6-foot-9 frame often against his former team in the first period, banging against a number of Senators players and nearly getting into a fight with winger Brian McGrattan. The pair were sent off for roughing after dropping their gloves and tussling in front of the Ottawa net.
Thomas had the best stop of the night early in the second period when he made a diving glove save, coming across the crease to rob Heatley.
Notes: Ottawa defenceman Andrej Meszaros assisted on Heatley's goal, giving the team's defencemen 16 points in the last four games. ... The Bruins entered the night last in the NHL in penalty killing. ... Chara's goal was the first power-play goal allowed by the Senators on the road this season.