The American's three-stroke victory at Muirfield on Sunday means he has completed the third leg of the career grand slam, having won the Masters and US PGA Championship a combined four times.
That leaves only the one he covets most the US Open, where he has finished as the runner-up a record six times, including last month at Merion Golf Club.
In winning the British Open, Mickelson joined Seve Ballesteros, Peter Thomson, Byron Nelson, JH Taylor and James Braid as a five-time major winner.
Only Ben Hogan, Gene Sarazen, Gary Player, Jack Nicklaus and Tiger Woods have completed a modern career grand slam of winning the Masters, US Open, British Open and PGA titles.
Nicklaus and Woods have won the full set two or more times apiece.
Mickelson, 43, is aware of where he stands in the pantheon and is set on sealing his legacy by completing his set of majors.
"I think that if I'm able to win the US Open and complete the career grand slam, I think that that's the sign of the complete great player," he said. "And I'm a leg away. And it's been a tough leg for me. But I think that's the sign. I think there's five players that have done that.
"And those five players are the greats of the game. You look at them with a different light. And if I were able to ever win a US Open, and I'm very hopeful that I will, but it has been elusive for me. And yet this [British Open] championship has been much harder for me to get."
How Mickelson has managed not to win a US Open is a mystery to many, including himself, after so many near misses, especially at Winged Foot in 2006, when he had a one-shot lead teeing off on the 72nd hole but finished with a double bogey to hand the title to the Australian Geoff Ogilvy.
"You have to be resilient in this game, because losing is such a big part of it," Mickelson said.
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