Garnett made 7-of-11 from the field and converted all six of his free throw attempts in pushing Boston to its sixth straight victory. The Celtics are 9-1 at home this season.
Paul Pierce contributed 12 points and Rajon Rondo had 19 assists.
Derrick Rose scored 20 points to lead the Bulls, who never led and lost for the fifth time in eight games.
Boston had only a one-point lead a minute deep in the second half, but finished three quarters with a 15-point advantage.