Whistled for a costly illegal pick late in a Game 2 loss, Garnett crushed the Sixers early and never let them think about a fourth-quarter rally.
Garnett scored 13 of Boston's 32 points in the second quarter and the Celtics became the first team to win by double digits. Game 1 and Game 2 were each decided by one point.
Rajon Rondo had 23 points and 14 assists. Paul Pierce, playing with a banged-up knee, had 24 points and 12 rebounds.
Game 4 is Friday in Philadelphia.
Garnett had somehow become forgotten in Boston's offense in Game 2 until the fourth quarter. Coach Doc Rivers said the Celtics simply weren't going to the 16-year veteran because they had established an offensive presence in the low post.
The Celtics wouldn't let that happen again.
They needed Garnett at his best in Philadelphia, where the Sixers had won their last four postseason games.
So much for that minor streak. Garnett yapped his way down the court after several big early buckets and clearly enjoyed taking it to the Sixers. He buried those 10 to 16 footers with ease in the second quarter to turn a seven-point deficit into a 13-point lead.
The Celtics again heard the whispers that they were too weary, too old to have their championship experience matter against the up-tempo Sixers.
Pierce is gutting out an MCL injury in his left knee that has robbed him of his jumper and slowed him down on both sides of the ball. He scored only 21 points combined in the first two games and failed to be the impact player the Celtics needed if they want to play deeper in the postseason.
He charged the lane in the first quarter for a couple of angry-looking dunks. He even pounded the backboard for emphasis after one as if to show the Sixers he still had some lift in those legs.
He'll need to do it again to hold off the Sixers.
Thaddeus Young scored 22 points and Jrue Holiday had 15 for the Sixers. Lou Williams and Jodie Meeks each scored 13. Starters Elton Brand, Spencer Hawes and Evan Turner combined for only 11 points.
Wearing their matching red 76ers logo T-shirts, fans fled for the exits at the 6-minute mark and the Sixers down 101-76.
The Sixers hadn't hosted a second-round game since 2003, when coach Larry Brown and All-Star Allen Iverson ruled the town. Julius Erving walked out to a roaring ovation when he presented the game ball and Eagles quarterback Michael Vick watched from a suite.
The Sixers hoped all the stars and hoopla that helped them knock off top-seeded Chicago would work again.
Back to the drawing board. Young scored three baskets and the rest of the Sixers had only two in the decisive second quarter.
Garnett tortured them from long range, toyed with them from inside, and got some deserved rest on the bench in the final minutes wearing a long-sleeve shirt.
Garnett was whistled for a critical offensive foul late in Game 2 on a potential game-tying possession for the Celtics. All seemed forgiven by the time the Celtics raced to a 25-point lead.
Rivers kept Rondo, Pierce and Ray Allen in the game until the final minutes even though the game was well out of reach. The Bulls were burned by that in the opening round in Game 1 when they left Derrick Rose in with a 20-point lead. He tore his ACL and was lost for the season - and the Sixers rolled to a series win.
Notes: Eagles owner Jeffrey Lurie and several other Eagles were at the game. ... Former Sixers owner Ed Snider watched the game from a courtside seat. ... Rivers was named to the NBA's Competition Committee. He said he had no idea until he received a letter saying he had been selected. ... The Celtics missed their first seven shots of the game, then made their next five.