Garnett put up 24 points and had 10 rebounds, while Paul Pierce knocked down the go-ahead three-pointer in overtime to help deliver the road victory.
"That's what it's all about. Finding a way to win," Pierce said. "Not feeling sorry for ourselves late about giving up a pretty good lead. And just finding a way to grind it out. And that's what we did."
Pierce finished with 23 points while Rajon Rondo had 15 and 16 assists, marking the 37th consecutive game he has registered at least 10 helpers. Rondo also added nine rebounds to fall one shy of a triple-double.
Boston (8-6) trailed by seven midway through the fourth quarter before going on a run that was capped by Garnett's hot streak. In overtime, Pierce snapped a 108-108 tie with his three-pointer and the Celtics (8-6) did not look back.
J.J. Redick led Orlando (5-8) with 21 points off the bench and Jameer Nelson added 20.
The Magic had trailed by 11 points in the third quarter before Nelson led a fight back.
The veteran point guard strung together the final eight points of the quarter to give the home team an 82-80 advantage going into the fourth.
"We put ourselves in a position to win, we just didn't close," Nelson said. "It's not on any one person, but we have to go out there together and really be focused and execute. And that's what they did in the fourth quarter and overtime."
Orlando forward Glen Davis had 15 points and seven rebounds against Boston, where he spent the first four years of his career.
The Magic, who traded All Star Dwight Howard to the Los Angeles Lakers in the offseason, had won two in a row entering Sunday's game.
The Celtics have now won two straight and trail Atlantic leaders New York Knicks by two games.
- Sports & Recreation
- Paul Pierce
- Jameer Nelson
- Rajon Rondo