The Bruins handed the Maple Leafs their first loss in regulation this season. Toronto had opened the campaign with a 4-0-1 record.
Toronto scored the first and last goals of the game but Boston ran off six unanswered scores in between and benefited early from the man advantage.
Horton tied it at 1-1 midway through the opening period with Matt Frattin in the penalty box for interference. Chara's goal came less than 6 minutes later while Joffrey Lupul was off the ice for tripping.
The win went to Tim Thomas, the hero of last year's Stanley Cup season, who made 26 saves.
All the Boston scores came against Jonas Gustavsson, who made his first start of the season.