Julius Boros (1973 US Open), Greg Norman (2008 Open) and Tom Watson (2009 Open) all came up short in their bids to achieve that feat holding 54-hole leads, but Mickelson wasn’t going to let his opportunity slip away in the 103rd US PGA Championship at Kiawah Island.
In a last-day effort that was every bit as impressive as the one he produced to prevail in the 2013 Open at Muirfield, the American had the crowds hooping and hollering from start to finish as he repeated a 2005 success in this event to join Lee Trevino and Nick Faldo as a six-time major champion.
“I mean, this is just an incredible feeling because I just believed it was possible and yet everything was saying it wasn’t,” said Mickelson of creating history by eclipsing the aforementioned Boros, who was 48 when he won this event in 1968.
“I hope that others find that inspiration. It might take a little extra work, a little bit harder effort to maintain physically or maintain the skills, but gosh is it worth it in the end and I am so appreciative to be holding this Wanamaker Trophy.
“I just love this game of golf. I love what I do. I love the challenge of competing against such great players. It was a really difficult task for me to go head-to-head with Brooks Koepka and come out on top.”
In scenes similar to when Tiger Woods won the Tour Championship in Atlanta in 2018, fans swarmed out onto the 18th fairway after Mickelson had hit his second shot then had to make his way through them before rounding off his deserved win.
“I don’t think I’ve ever had an experience like that, so thank you for that,” he said at the presentation ceremony to loud cheers. “Slightly unnerving, but exceptionally awesome.”
Oosthuizen (73) and Koepka (74) shared second spot on four-under, two ahead of Europe’s Ryder Cup captain Padraig Harrington and fellow Irishman Shane Lowry after they closed with matching 69s in the same group, as well as England’s Paul Casey (71) and American Harry Higgs (70).
Mickelson started out with a one-shot lead in his bid to create history, but an under-hit opening putt led to a bogey at the first and, with playing partner Koepka rolling in a 12-footer for birdie, he immediately fell one behind.
Koepka wasn’t in front for long, though, as he undid that good start by running up a double-bogey 7 at the second, where Mickelson made birdie, then missed a short par putt at the third.
Mickelson also dropped a shot at the third before raising the biggest roar of the week as he holed out from the sandy waste area for a brilliant birdie-2 at the fifth.
Koepka responded with a majestic approach at the next to set up a birdie only to give that shot back straight away, losing ground on Mickelson again at a par 5 as the left-hander converted a 10-footer for a birdie.
Mickelson closed the front nine with two solid pars to be out in 36, level-par, having hit just two out of seven fairways and six of nine greens in regulation.
What mattered most, though, was that he had doubled his overnight advantage and that quickly became four shots, helped by Oosthuizen starting for home with a bogey in the game ahead but more so how the final pair played the 10th hole.
Mickelson split the fairway with one of his best drives of the week, followed it with a majestic second and rolled in a 10-footer, celebrating his 21st birdie of the week with a fist pump.
In contrast, Koepka, who was starting to become very ragged, missed the fairway, came up short with his approach and took 5 before following that with a 6 at the 11th - his third sore one of the day at a par 5.
That double disappointment dropped the two-time winner five behind, but Mickelson’s lead was down to three when Oosthuizen ignited his challenge by making a birdie at the 12th.
Unfortunately for the 2010 Open champion he followed that by taking a double-bogey 6 at the 13th, where he was left off the tee then found the water on the right of the green with his third, meaning Mickelson had a five-shot cushion but not for long.
He’d dropped two shots at the 13th the third round after being wet off the tee. That part was negotiated safely on this occasion only to see his approach topple into the hazard.
A resultant bogey was not disastrous, but another dropped shot at the par-3 14th was a definite sign that he was starting to get a bit edgy.
Oosthuizen shaved the edge of the hole with an eagle attempt at the 16th but closed the gap to just two shots with the birdie, which Mickelson then matched before giving that shot straight back after finding a nasty spot through the back of the 17th green and having to take his medicine.
That meant Mickelson headed to the last with a two-shot advantage over both Oosthuizen and Koepka, who was back hanging on to the leader’s coat-tails after finding his stride again to make birdies at the 15th and 16th.
Mickelson’s closing tee shot missed the fairway on the left, but it was job done after safely finding the putting surface, with fans spilling out onto the fairway as he made his victory walk up to the green to chants of “lefty, lefty, lefty”.
The new champion shared a warm embrace with his brother and caddie, Tim, after tapping in for a closing 73 to finish with a six-under-par 282 total.
He walked off the green giving a thumbs up to the fans and was congratulated on the way to signing his card by Jon Rahm, Rickie Fowler, Paul Casey, Padraig Harrington and, finally, Oosthuizen.
“This is just amazing,” admitted Mickelson. “I believed I had the ability to do it. I have not played the way I knew I could. I’ve been frustrated but, with the help of my wife Amy, my brother, (coach) Andrew Getson, (manager) Steve Loy, I’ve been able to get back to playing golf at the highest level and it is so fulfilling and rewarding and this is a moment I will cherish forever.”
Martin Laird birdied the last for a closing 72 to finish on one-over, earning the Denver-based Scot a share of 23rd spot - his best finish in this event - with Bob MacIntyre (73) ending up joint-49th on five-over.