Garnett backed into the lane one-on-one against Jared Jeffries and executed a perfect jump-hook shot to put the Celtics ahead for good at 94-93.
On the Knicks' ensuing possession, he deflected a Jeffries pass under the Celtics' basket and fell on the loose ball, sealing Boston's second straight home victory over New York in their Eastern Conference quarterfinal series.
Garnett finished with 12 points and 10 rebounds while Rajon Rondo led the Celtics with 30 points and seven assists. Paul Pierce added 20 points and Ray Allen provided 18 for Boston.
Carmelo Anthony poured in 42 points and grabbed 17 rebounds for the short-handed Knicks, who lost Amare Stoudemire to back spasms in the second quarter and were playing without the injured Chauncey Billups.
New York will host Game 3 Friday.