Mickelson tied Mark O'Meara's record with his fifth victory in the AT&T Pebble Beach, the first one also a Monday finish in 1998 because of bad weather, with one big difference - that Monday finish was more than six months later in August.
Red-hot Mickelson and Casey had opposing views on whether to continue as darkness fell, Mickelson putting out for a par at the 16th and saying he was ready to head to 17 as Casey opted to call it a day with a three-foot putt for par in front of him.
US PGA Tour officials suspended play for two hours when heavy rain, wind and even hail lashed California's Monterey Peninsula, with the postponement all but guaranteeing the final round will have to be completed on Monday. Mickelson never came close to making bogey and won for the 44th time on the PGA Tour.
The American is on 18 under with two holes left to play, with playing partner Paul Casey on 15 under.
I've received stick for years for backing Mickelson repeatedly in tournaments he likes but it's paid off yet again and I've already backed him for this week's Genesis Open too. I'm very happy we won the team competition.
The victory, Mickelson's first since the WGC-Mexico Championship last March and first on US soil in six years, carries special significance for him.
Monica Witt, former Air Force counterintelligence specialist, accused of spying for Iran
Later that year she worked with Individual A to make a video critical of the United States, and the two continued to communicate. In Iran, she has appeared to work at times to publish anti-American articles for the state-owned Press TV.
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.
Third-round leader Paul Casey, who mixed two bogeys with two birdies, was at 15 under with three holes remaining, tied with Scott Stallings who finished with a 66. He had opened the week with a 69 at Pebble Beach, but that was in much nicer conditions that what he and the rest of the field faced on Sunday.
"Very simply it's nothing like the course we'll see".
Phil Mickelson tips his hat on the 18th green after winning the Pebble Beach Pro-Am. "And to have my pro career start here, to have this victory, means a lot".
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.
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. I would love nothing more than to add to it five months from now, but that's so far down the road.