The Celtics won their fifth straight road contest, getting 20 points from Paul Pierce and 12 assists from Rajon Rondo.
Rodney Stuckey added 19 for Detroit, which has dropped three out of four.
Boston never trailed, but its lead was trimmed to four with less than 6 minutes to play. Glen Davis then made a layup, Eddie House hit a three-pointer and Garnett made a jumper to boost the Celtics' advantage to 78-67 with 4 minutes to go.
Boston's Kendrick Perkins was ejected from the game midway through the fourth period after grabbing Detroit's Jason Maxiell in a headlock and yanking him to the floor.