Carpenter went 4 for 5 with four RBIs after moving back to the leadoff spot. It was his first career multihomer game and he has a career-high 12 home runs this season.
Jhonny Peralta's two-run single off Rockies closer John Axford tied the game at 8-all and Garcia's walk scored Kolten Wong as the Cardinals recorded their seventh walk-off win of the season.
Axford (3-5) took his third loss in his last four outings.
Cardinals starter Carlos Martinez gave up five runs on 10 hits in five innings. Carlos Villanueva (4-3) earned the win.
