Poston, who lost his lead after 12 holes, made a 7-foot birdie putt on the 18th hole to salvage an even-par 72 and force a playoff with Ryan Gerard, and then won on the second extra hole when Gerard missed a 6-foot par putt.
Gerard made a 40-foot birdie putt on the 17th hole to briefly take the lead, closed with a 68 and didn't really make a mistake over the final four hours until the final putt.
So ended the longest day for both of them - 33 holes, including 13 in the morning to complete the third round and two playoff holes.
But what a payoff for Poston in so many ways. He had not finished in the top 20 in his previous 13 tournaments this year. And then he delivered the goods and celebrated with a handshake with tournament host Jack Nicklaus.
"A lot of holes, a lot of grit," Poston said. "Obviously, didn't play my best first 12, 13 holes, but I told myself I knew I was going to shake Jack's hand walking off 18, and I wanted to be proud of that handshake regardless of how it turned out. So I'm thrilled it happened this way."
The perks went beyond the $US4 million prize. Poston earned a spot in the next three majors with one great week - high enough in the world ranking to avoid a 36-hole qualifier Monday for the US Open, the one British Open spot available this week, and a return to the Masters.
They finished at 12-under 276 after Poston and Gerard pulled away from what had been a five-way tie for the lead with an hour to go in the tournament.
Tommy Fleetwood drilled a fairway metal to 5 feet for eagle on the par-5 15th. Wyndham Clark made a late surge. Sam Burns was never too far behind. All of them were at 11 under heading to the final few holes.
Clark, who made birdie on the 16th to reach 11 under, closed with two pars for a 67. He wound up alone in third, one shot out of the playoff.
Fleetwood hit into the rough with his first three shots on the 17th and had to scramble for bogey. He shot 68 and tied for fourth with Burns who also missed the 17th fairway, before his next shot tumbled back down the rough and settled on the bridge over a small creek as he took bogey.
Then lone Australian to make the halfway cut, evergreen Adam Scott rattled home with a four under 66 to tie for 12th, six shots outside the playoff.
Scottie Scheffler, trying to join Tiger Woods with a third straight victory at the Memorial, was never in the mix for so much of the week. He closed with a 71 and tied with Scott, but felt he did enough right over the weekend that his game was headed in the right direction.
His next stop is the US Open, where a victory would give him the career grand slam.
Rory McIlroy is now 0 for 14 at the Memorial. He birdied his first three holes before getting caught in dense rough that slowed his momentum. He shot 68 and also tied for 12th.
