Phil Mickelson is the oldest major champion in golf history after completing a storybook week amid wild scenes at the 103rd PGA Championship.

The 50-year-old held off Brooks Koepka and Louis Oosthuizen to win by two shots – a one-over-par 73 on Sunday giving him a six-under 282 total.

In scenes reminiscent of Tiger Woods’ epic return to the winner’s circle in 2018 at the TOUR Championship, spectators broke containment lines and flooded the Ocean Course’s 18th fairway before Mickelson could even reach the green.

It is the sixth major of Mickelson’s career but first since winning the British Open in 2013.

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By claiming his second PGA Championship 16 years after his first, Mickelson brooke the record held by 1968 PGA Championship winner Julius Boros (48) as the game’s oldest major champion.

“I mean, this is just an incredible feeling because I just believed that it was possible, but yet everything was saying it wasn’t,” Mickelson said.

Mickelson started the final round with a one-shot lead at the windswept course and despite three bogeys in his opening seven holes, he produced three stunning birdies to offset them.

The veteran will forever remember an incredible shot that energised the swarming galleries, and his own energy, on the par-three fifth.

After a tee shot landed in a sandy waste area the short-game wizard holed out his next shot for birdie.

When he birdied the 10th hole Mickelson was four shots clear of the challengers and while Oosthuizen pulled within three after a birdie on the 12th hole, South African fell apart with a double bogey a hole later.

Mickelson was five clear with six holes to play but induced some nerves with bogeys on the 13th and 14th holes.

A birdie on 16 but bogey on 17 meant he went to the 18th tee leading by two.

A brilliant approach to 16 feet from the left rough sealed the deal.

“The biggest moment of a legendary career, Phil defeats father time,” long-time golf announcer Jim Nantz said as Mickelson tapped in the winning putt.

Four-time major winner Koepka (74) and Oosthuizen (73) shared second at four-under.

Koepka birdied the first hole to lead outright but double-bogeyed the second to hand it right back to the veteran.

Back-to-back bogeys on 10 and 11 and another at 13 made his road to victory tougher.

Oosthuizen won the 2010 Open title 3,962 days ago. It remains his only major win and he now has five runner-ups in golf’s biggest tournaments.

Australia’s top-ranked player Cameron Smith raised slight hopes of a charge towards the top 10 with two birdies to open his final round but a bogey and double bogey soon after started a big slide to a 77 and seven-over finish.

European Tour player Jason Scrivener finished as the leading Australian with a three-under 69 lifting the Western Australian to one-over for the week, tied for 23rd in just his second major.

Former world No.1 Jason Day fell short of his quest to finish inside the top 17 of the tournament, the mark needed to qualify for the US Open.

A one-under 71 left him four-over and T44 for the week.

Matt Jones finished strong with a 68 to get to two-over and T30, Cameron Davis (72) finished up seven-over with Lucas Herbert (72) nine-over.

-AAP