In winning at aged 50, Mickelson surpassed the record previously held by Julius Boros, who won the PGA Championship in 1968 at age 48.
The victory is the sixth major title of Mickelson’s long and storied career, with his previous PGA Championship coming 16 years ago in 2005.
Heading into the final day with Mickelson just a shot ahead of Koepka, the competition tightened up even more in the early holes of Sunday’s round.
And a bogey for Mickelson and birdie for Koepka on the opening hole tied the scores up, with 17 holes standing between the pair and the famous Wanamaker Trophy.
The pair went back and forth, exchanging birdies, pars, bogeys and the odd double bogey, during the first four holes, with little to separate the pair.
But the Mickelson magic that fans have come to love over his career really began to emerge on the par-three fifth hole.
In a bunker off of his tee shot, the American golfer chipped into the hole, sending the watching spectators into a rapture and giving them reason to believe that history could be in the making.
And after a bogey on the sixth, Mickelson’s resolve seemed to strengthen, and he looked to be having the time of his life.
While Koepka’s confidence seemed to drip away hole by hole, Mickelson only got better, birdieing two and picking up pars on the next six holes.
There was still time for one heart-in-mouth moment for Mickelson, with his second shot on the 13th dropping into the water. However, after a few errant shots from Koepka, he wasn’t made to pay the price with both players bogeying the hole.
On the next hole, Mickelson’s tee shot left him a trick chip onto the green. And it cost him, as he picked up consecutive bogeys, reducing the gap between him and Oosthuizen to just three.
And just when all the pressure was on — in the group ahead, Oosthuizen’s birdie reduced the gap to two shots — Mickelson produced a 366-yard drive on the 16th hole, the longest drive on that hole by any player all week.
It resulted in an all-important birdie, once again stretching his lead to three shots, which nervously was reduced to two, moments later.
In an amazing scene on the final hole, however, surrounded by thousands of fans and bathed in sunlight, Mickelson two-putted his name into the history books.
For Koepka, who was seeking his third PGA Championship title in four years, struggles with his putter were the decisive factor that held him back.
And although he pushed Mickelson all the way, the four-time major winner described being “super disappointed” and “pretty bummed” in the immediate moments after the final round.
“I’m not happy. I don’t know if there’s a right word I can say on here without getting fined, but it hurts a little bit,” he told the media in his post-round press conference.
“It’s one of those things where I just never felt comfortable over the putts. I don’t know why, what happened.”
Heading into the PGA Championship, Mickelson was most likely very far away from people’s favorites to win.
After the first round, he sat amongst the leaders, just three shots off Corey Conners at the top.
But he made moves after that, finishing Friday and Saturday atop the leaderboard.
In winning at 50, he breaks Boros’ record of 48 years, 4 months, 18 days of the oldest major winner in golf’s history. Golf legend Jack Nicklaus won his last major at 46.
And before Sunday’s victory, he’d only won twice on the PGA Tour since his last major victory in 2013 as well as two wins on the PGA Tour Champions in 2020.
In winning, Mickelson became the fourth golfer in PGA Tour history to win in four different decades. He joins Sam Snead, Raymond Floyd and Davis Love III.
More to follow