Poston never trailed since making a 20-foot birdie putt on the 14th hole as he returned early on Sunday to finish the third round with a 66 and a three-shot lead.
He backed it up with a closing four under 67 but still had to sweat it out till the very last putt to win from fellow American Doug Ghim.
Ghim closed with a 65, making a six-foot birdie putt on the final hole at the TPC Summerlin to close within one shot. Poston lagged his 45-foot birdie putt to four feet and calmly sank that for victory at 22-under 262.
The winning putt felt a little longer because Poston missed a six-foot birdie putt on the 16th and a four-foot par putt on the 17th, either of which would have put the tournament away.
"I just told myself, 'This is what you dream of, a putt to win on the PGA Tour. Try to forget the last two,'" Poston said. "I told myself, 'I've made a million of these. Just do one more.'"
The victory assures Poston of returning to the Masters for the third straight year and moving back inside the top 50 in the world. He was playing for the first time since the BMW Championship, wanting to spend time at home with a daughter born in March.
This was a happy return for the 31-year-old from North Carolina, who already was guaranteed being in the $US20 million signature events because he finished among the top 50 in the FedEx Cup.
Ghim's runner-up result was his best on the PGA Tour.
Germany's Matti Schmid continued his strong play in the FedEx Cup Fall, closing with a 66 to tie for third with Rico Hoey of the Philippines, who also had a 66.
Schmid was No. 125 in the FedEx Cup - the top 125 keep a full PGA Tour card for 2025 - when he tied for 16th in Mississippi, finished fifth last week in Utah and tied for third in Las Vegas. He now is up to No. 77.
Another big consolation prize went to Michael Kim, who closed with a 62 and tied for fifth, moving from No. 129 in the FedEx Cup to No 112.
The PGA Tour next heads to Japan for the Zozo Championship, a 78-man field with no cut, before returning to North America for three tournaments to close out the season.
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