The West Waterford man (35), who only grabbed his first win in Kentucky last year after coming through the eGolf mini Tour and the Korn Ferry Tour to establish himself at the top level, closed with a one-under 70 to win by a shot from a fast-finishing Thomas Detry on 19-under par.
"It's a completely different feeling, but just as special," Power said of a win that catapults him one place ahead of Rory McIlroy to fifth in the FedEx Cup and to a career-high of 32nd in the world in his 138th PGA Tour start.
"It's so hard. You play a lot of tournaments, and it's only my second win, and I don't know how many events. It's very special. It's going to take a little while to sink in, but I am absolutely over the moon."
Power found himself two shots behind playing partner Ben Griffin with seven holes to go but played them in two-over to Griffin's six-over to get the job done.
"This course is always going to be a tale of two sides again. You were always going to make some birdies on the front and Ben made a lot, and after getting 10 and 11, I knew it was going to be really hard coming in," Power said.
"I made really hard work of it in the end, but I'm delighted to get it done. Obviously, Ben was a great guy to play with, and it's always nice to see putts going in, but I am absolutely delighted."
Just 469 days after his breakthrough win after a six-hole playoff for the Barbasol Championship in Kentucky, the West Waterford man (35) closed with a one-under 70 to win by one stroke from Detry on 19-under par.
It was a hard-fought win for Power, who started the day tied for the lead with rookie Griffin on 18-under par.
But while Griffin played his first 11 holes in five-under to go two clear of Power with seven holes to go, he collapsed down the stretch and shot 72 to finish tied third on 17-under with Patrick Rodgers and Kevin Yu.
Power also had his struggles, and while he was two clear playing the last, he spun his approach with a wedge off the green and 30 yards down the fairway before two-putting from four feet for bogey and victory.
The highest-ranked player in the field at 48th in the world, Power picked up a cheque for $1.17m to move to fifth in the projected FedEx Cup standings ahead of McIlroy and to a career-high of around 32nd in the world, just behind Dustin Johnson.
He's also set to move into the top 10 in the race for three spots via the Ryder Cup World Points list for Luke Donald's European team for Rome after a nerve-wracking final day in Hamilton.
While Power and Griffin birdied the first, Griffin birdied the par-five second to edge clear, then hit a sensational approach to three feet at the fourth to go two strokes clear on 21-under.
Power rattled in an 18-footer at the fifth to cut the gap to one but missed an 11-foot chance at the sixth where Griffin made another birdie to regain his two-shot advantage.
The West Waterford man chipped close at the par-five seventh to get within one again with a birdie, then lagged a 40-foot putt stone dead at the eighth, where Griffin three-putted to draw level.
They turned three shots clear of the field on 21-under par, but while Griffin birdied the 10th and 11th to go two ahead, he bogeyed the next four holes and double-bogeyed the 16th.
While Power also bogeyed the shot 13th, he bashed in a 25-footer for a birdie at the 14th for birdie to take the lead as Griffin hooked a driving iron into the trees and made a bogey.
The two-shot swing saw Power go one shot clear, but while he mishit his tee shot at the 15th and could not get up and down from 57 yards for par, Griffin also dropped a shot after bunkering his approach.
Griffin then hit his approach into the hazard at the 16th and made a double bogey, but while Power almost holed a 100-foot putt for birdie, he missed the return from six feet and found himself just one ahead of Detry, who holed a bunker shot at the last to set the target at 18-under with a 67.
Power got up and down from greenside rough for a birdie at the par-five 17th to go to the last two clear on 20-under. But he would only win by one in the end as he spun his approach off the green, 30 yards down the fairway and eventually missed a five-footer for par.
It was still the sweetest two-putt bogey of his life and a win that opens the door to Ryder Cup glory next year.
He's now exempt until the end of 2025 and earns spots in the Sentry Tournament of Champions, The American Express, The Genesis Invitational, the Arnold Palmer Invitational, The Players Championship, the Masters Tournament, the RBC Heritage, the PGA Championship, the Charles Schwab Challenge and the Memorial Tournament.
On the Algarve, Jordan Smith ended his five-year wait for his second DP World Tour title with an impressive three-shot win in the Portugal Masters at Dom Pedro Victoria in Vilamoura.
The Englishman (29) finished on 30 under par, carding rounds of 62, 67, 62 and 63 to win from Malaysia's Gavin Green on 30-under par.
"It's just amazing," said an emotional Smith. "It's been a long grind for those five seasons, just pushing it out and finally getting over the line today. Over the moon.”