Mickelson, ranked No 115 in the world before this week, delivered a win for the ages by firing a final-round one-over par 73 to finish 72 holes on 6-under par 282 with four-time major winner Brooks Koepka and South African Louis Oosthuizen second on 284.

A surreal scene at the 18th hole saw spectators surround the green and chanting “Lefty” as he two-putted for par to complete a record-breaking victory for the ages.

Just last week, Mickelson accepted a special exemption to play in the US Open in June, where he will be going for a career grand slam.

“This is just an incredible feeling,” Mickelson said. “I just believed it was possible but everything was saying it wasn’t.

“I hope others take that inspiration. It might take working on your skills but, god, is it worth it.”

Mickelson, an $80 outsider to lift the Wanamaker Trophy at the start of this week, was mobbed by spectators as he headed up the 18th fairway, where he would seal his first major win since his triumph at the 2013 Open Championship.

Always a crowd favourite, Mickelson was roared onto the green to chants of “Lefty, Lefty” before tapping in for a famous victory.

“I don’t think I’ve ever had an experience like that, so thank you for that,” Mickelson told the crowd as he accepted the prize.

“Slightly unnerving, but especially awesome.”

The crowd, limited to 10,000 people by Covid-19 safety measures, roared with every successful swing by Mickelson, creating an electric atmosphere all day on the punishing Ocean Course, the longest layout in major history at 7,876 yards.

Additional reporting by Agence France-Presse