Garcia appeared as a substitute for Gareth Williams in the 73rd minute, and it didn't take him long to make a crucial impact. In fact, the former West Ham forward broke the deadlock with his first touch of the ball to put Colchester ahead on 74 minutes.
Greg Halford's long throw was flicked on by Chris Iwelumo and the alert Garcia stabbed home from close range.
Port Vale, who were unbeaten in their previous four matches, were handed a lifeline in the 79th minute when referee Phil Joslin awarded them a penalty for handball against Wayne Brown.
Vale striker Michael Husbands stepped up to crash home the spot-kick and get the visitors back on level terms.
But Colchester have got into the winning habit in recent months and they came up trumps with an 87th-minute decider, thanks to a second close-range effort from Garcia.
This time it was a free-kick propelled into the box by Kevin Watson. Iwelumo again met it with a snapshot that keeper Mark Goodlad did well to block. However, Garcia was in the right place to nod home the rebound and secure the points.
Phil Parkinson's men have now won 16 of the last 18 matches, a remarkable record.
However, this latest win came at a cost. Goalkeeper Aidan Davison limped off with a strained hamstring midway through the second half and worse was to follow, as midfielder Watson was stretchered off with what looked like a serious knee injury in the last minute of normal time.
Watson was felled by a late challenge from Vale midfielder Sam Togwell, who was lucky to stay escape with a lecture after being booked earlier in the game.
The visitors felt aggrieved themselves because they believed that Chris Birchall's in-swinging corner, after 48 minutes, had crossed the line after Davison lost his balance with the ball in his hands.
Davison hastily threw the ball away and play carried on with the referee ruling that the ball was not wholly over the line.