Mickelson is now top of the leaderboard on 18-under with two holes left to play, while Casey is on 15-under with three holes left.
"Sometimes I just get in my own little bubble and I don't see the big picture and I just, I wanted to finish, I was playing well, and I just didn't take all the factors into account", he said after Monday's two-hole finish. He drilled a 7-iron into eight feet on the par-3 17th and made par, and then played conservatively up the par-5 18th and finished with a 6ft birdie for a three-shot victory.
The American closed out a bogey-free 65 to finish on 19 under and finish off a three-shot win in California, as Casey made a final-hole birdie to grab outright second ahead of Scott Stallings.
"This was the first event for me as a pro".
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.
Mickelson said he could "see just fine" even after sunset, knowing darkness falls quickly on the Monterey Peninsula.
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Lost in the late start - one hour delay from rain, a two-hour suspension from hail - was sheer brilliance from the 48-year-old Mickelson who didn't come remotely close to making a bogey and was on the brink of a fifth victory in the AT&T Pebble Beach Pro-Am. Casey said there was no way to finish and they had to return Monday morning.
That also led him to enter this week's Genesis Open, which he had not planned to play, prior to defending a title at the WGC-Mexico Championship next week.
Pebble Beach was so soft that balls were plugging in the fairway when they landed.
His first one also was a Monday finish, when players returned in August - more than six months later - to finish the third round of the weather-plagued tournament to make it official.
Instead, he had to return early Monday morning to complete his 44th PGA Tour victory, claiming a fifth AT&T Pebble Beach National Pro-Am while also becoming the eighth-oldest victor in PGA Tour history.
"We've got two tough holes left and I don't want to jump the gun and get past that", Mickelson said.
For now, he was glowing over another victory that keeps him as relevant as ever. And it is where he will turn 49 on the final day of the 2019 U.S. Open.