Mills, who came off the bench to score 25 against the Clippers Tuesday, to lead the veteran Spurs in shots taken and pace them to their sixth win on a nine-game road trip that has turned into more of a pleasure cruise.
Belinelli sank the go-ahead 3-point shot with 1:34 to play and added a pair of free throws with 2.2 ticks remaining to close out the victory.
Damian Lillard led Portland with 31 points and Mo Williams added 19. Portland lost its third consecutive game. Wesley Matthews scored 18, including a nifty steal-and-dunk play with 1:51 remaining in the game that tied the score 101-101.