Mickelson had a three-shot lead over Paul Casey with two holes to play when it was too dark to finish Sunday night (Monday NZT) - no matter how hard Mickelson lobbied to keep going - because of delays from rain and a hail storm.
Casey returned to the fray today, tapped in his par save at 16 and then preceded to par the par three 17th and birdie the par five 18th but it wasn't enough to reel in Mickelson who also finished 3-4.
Mickelson briefly went three ahead after converting from eight feet at the 13th, before picking up a sixth birdie of the day at the par-five next to match Casey's 15-footer from the fringe.
Casey birdied the 18th for a 71 to finish alone in second, and he won the pro-am portion of the tournament.
The five-times major champion will get a chance to complete the career grand slam when the U.S. Open is played at Pebble Beach in June, but Mickelson wants to savour his latest success first.
Thirteen of Mickelson's 43 PGA Tour victories have come in his home state of California.
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"I thank Paul for having us come back out here this morning", Mickelson said. "The rough will be so deep and the greens will be so fast, it will be a totally different course". Before he won, he marked his 500th PGA Tour cut made on Friday.
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.
American Scott Stallings was in the clubhouse on 15-under after a six-under par 66 with Jason Day of Australian and South Korean Kim Si-woo tied for fourth on 13-under. He won the British Open that summer in Scotland, and the Mexico Championship past year.
Asked in a greenside interview what the performance did for his confidence, Mickelson said: "Absolutely nothing, because it's nothing like the course we'll see".
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.
This was Mickelson's fifth AT&T title so it's fair to say he was a tournament specialist before this week but he was still generally a [36.0] chance and he even hit a high of [50.0] before the off. Casey, who traded at a similar price to Lefty pre-event, hit a high of [80.0] and a low of [1.5] in-play.
For years, Mickelson, 48, has rebuffed talk of his age, choosing to focus on aspects that encouraged him going forward even through a four-year stretch without a victory.