Garcia won the Wyndham Championship in North Carolina by two shots after the tournament spilled into a Monday finish because of heavy weekend rain.
He resumed one in front with 14 holes to play, but stood on the 13th tee sharing the lead with American Chad Campbell and South African Tim Clark.
Then came birdies at four of the next five holes and a closing bogey secured the 32-year-old Spaniard his eighth PGA Tour victory, but first since the 2008 Players Championship.
Poulter had moved into the 10th and last automatic qualifying spot with his joint third place finish at the USPGA Championship, but looks a certainty for one of captain Jose Maria Olazabal's two picks next Monday.
Belgian Nicolas Colsaerts is favourite for the other wild card, even more so after a joint seventh place finish at the Sedgefield Country Club in Greensboro.
Given a special invitation to the event, the 29-year-old was another of the 38 players unable to finish on Sunday and instantly made birdies at the 13th and 14th on his return.
They lifted Colsaerts into a tie for third, but after he closed with four pars for a 66 - the same score as Garcia - he slipped back down the leaderboard.
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Unlike Garcia and Poulter, who are involved in the start of the FedEx Cup play-offs, Colsaerts still has this week's Johnnie Walker Championship at Gleneagles to come and a third place finish there would take him ahead of German Martin Kaymer, the player now in the last automatic spot.
England's David Lynn, the surprise runner-up in the final major of the season, and Spain's Rafa Cabrera Bello also have a chance in Scotland to earn a cup debut, but they each need to win.
Garcia began his final burst with a superb 136-yard approach to within 18 inches of the flag on the 13th, got up and down from sand at the long 15th, hit his tee shot to two feet on the 166-yard next and then fired in another iron to six feet at the 17th.
A four-shot lead came down to one when Clark holed out from just short of the green for birdie on the last and there was still work to be done when Garcia drove into the trees there.
He had to chip out and was still not on the green in three, but got down in two more for an 18 under par total of 262. Clark was second and American Bud Cauley alone in third one further back.
"I am proud of the way I played coming in," Garcia said.
Olazabal now knows nine of his 10 automatic qualifiers - Rory McIlroy, Justin Rose, Graeme McDowell, Paul Lawrie, Francesco Molinari, Luke Donald, Lee Westwood, Garcia and Peter Hanson.
The final spot is between Kaymer - not in the field at Gleneagles - Colsaerts, Lynn and Cabrera Bello.
Garcia, who failed to make the 2010 side and acted as one of Colin Montgomerie's assistants, added: "Obviously I won a couple of times last year in Europe, but it's been a while here (in the United States) and it feels great.
"I hit some really good shots and made some nice four or five-footers when I had to. I'm very happy about the way the week has gone."
He used a local caddie for the tournament because he wanted to see how making more decisions on his own worked out and said afterwards he is likely to continue doing the same for the remainder of the season.
As for how much David Faircloth will earn - regulars expect 10% of the winning cheque - Garcia said it would not be the same arrangement, but "he is going to be happy with what he gets".