Mickelson, who began the day with a one-shot lead over Koepka, put the finishing touches on one of the most impressive feats in golf with a one-over-par 73 at the Ocean Course that left him at six-under on the week.

By collecting the sixth Major of his career and first since the 2013 British Open, Mickelson surpassed Julius Boros, who was 48 years old when he won the 1968 PGA Championship, as the oldest Major winner.

After an eventful front nine that produced twists and turns at seemingly every hole, it was fan-favourite Mickelson who emerged two shots clear at the turn and soaking up energy from the limited spectators in attendance at the year’s second Major.

Mickelson moved three shots clear after a birdie at the par-four 10th and the final stretch was more a coronation than a dogfight as four-times Major champion Koepka and 2010 British Open winner Oosthuizen ran out of holes.

Mickelson, who led by five shots with six holes to play, did see his lead cut to two shots after bogeys at the 13th and 14th compounded by a birdie at the 16th by Oosthuizen, who was playing in the penultimate pairing.

But with victory in sight, an undeterred Mickelson showed great resolve and even flashed a thumbs up to the crowd as he went on to reclaim a three-shot cushion with a birdie at the 16th before a bogey-par finish.

Oosthuizen (73) and Koepka (74) finished in a share of second place while European Ryder Cup captain Padraig Harrington (69), Ireland’s Shane Lowry (69), Englishman Paul Casey (71) and Harry Higgs (70) were a further two shots back in fourth place.

Northern Irishman Rory McIlroy, who won the PGA Championship at Kiawah Island in 2012 and was a favourite this week, closed with an even-par 72 that left him at five over par.