Casey's only hope was for Mickelson to make a mistake on the closing holes, and there was little chance of that.
Whether it was continuing to be a part of U.S. Ryder Cup and Presidents Cup teams, finding a new workout routine, changing coaches, discovering a healthier diet, befriending - and needling - tour players who are young enough to be his son, Mickelson did his best to stay young.
Law trailed overnight leader Wade Ormsby by five shots going into the back nine of the Vic Open in Australia and by three with just three holes left to play but piled on the pressure with a birdie-par-eagle finish.
The last time the final round was played on Monday was in 2000, when Tiger Woods rallied from a five-shot deficit.
"I get exactly where Paul is coming from", Mickelson later said. In 2018, Ted Potter, Jr. prevailed over Phil Mickelson, Jason Day and two-time Pebble Beach victor Dustin Johnson.
Mickelson argued that he could "see just fine" on Sunday evening, moments after sunset with two holes remaining.
The weekend's PGA Tour event is still now ongoing and scheduled for a Monday finish due to adverse weather conditions over the weekend causing delays.
"I thank Paul for having us come back out here this morning", Mickelson said.
Scott Stallings, the American who held the clubhouse lead at nightfall on Sunday on 15-under, was on hand in case he might be involved in a playoff, but settled for third.
Mickelson's first goal is that fourth green jacket and he believes that even now, at the age of 48, he has the power and the putting touch to give McIlroy, Jordan Spieth and the rest a run for their money in eight weeks' time. He won the British Open that summer in Scotland, and the Mexico Championship a year ago.
"I really don't think there's any carryover from here to the U.S. Open", he said after the victory. The US Open remains the final piece missing for him to complete the career Grand Slam, though Lefty was quick to caution that this week had no bearing on this summer. Mickelson marked his ball all week with a silver coin given to him by his grandfather, Al Santos, one of the original caddies at Pebble Beach in 1919.