He played two holes just as well as the previous 16 and polished off his bogey-free final round with one last birdie for a 7-under-par 65 and a three-shot victory over Paul Casey in the AT&T Pebble Beach Pro-Am.
Overnight leader Casey had a three-shot advantage over Mickelson at the start of the day, but the five-time major victor made up the deficit with three birdies in the front nine and three more after the turn.
Casey resumed with a three-foot par putt at the 16th hole on Monday, while Mickelson had already completed the hole. Casey at least birdied the 18th for a 71 to finish alone in second, the difference of $152,000.
"But I have pretty good vision, I can see fine and I'm playing well so I wanted to continue, that's all there is to it".
"Every time I get here I have such feelings of gratitude that all that this place has done for my family, starting with my grandfather", he said.
Mickelson, seen shaking his head when the horn sounded Sunday night, said he thanked Casey on Monday morning for holding his ground because it was fair to both of them.
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The course will be completely different in June, with the rough taller and the greens firmer and no amateurs to take the edge off the proceedings.
Mickelson claimed his 44th US PGA Tour title and has also won three European Tour titles, the 2004 Grand Slam of Golf and 2004 Skins Game, and a 1993 Challenge Tour event in Paris.
A birdie from the fringe at 14 left him even for the day and 15-under par for the tournament.
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. He won that summer's British Open at Muirfield and last year's Mexico Championship.
"(It means) absolutely nothing because it's nothing like the course we'll see", Mickelson said.
Mickelson and Casey were waiting to tee off when clouds moved in quickly moved in, and rain turned into hail that pounded umbrellas, many of them held sideways to account for the wind. 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.
Some players finished in the dark with no chance of winning, but showed the effect of playing without light. Jason Day (68) and Kim Si-woo (68) were two strokes further back on 13-under 274.