He just couldn't beat Sunday's setting sun. Dustin Johnson won the first of his two titles in 2009 when the final round was delayed until Monday because of rain, and then canceled because the storm system wouldn't leave.
Mickelson at least wanted to try.
Mickelson already made his par on the 16th and was 6 under for the day, with no bogeys on his card, and 18-under par for the tournament.
"I can see fine", Mickelson insisted before finally relenting to come back Monday and finish up the final two holes. Play was expected to resume at 1 p.m. local time. "We've got to finish it off, so I don't want to jump the gun and look ahead [to a potential win]".
"I know a lot can happen in these two holes, and they have happened in the past", Mickelson said. "We had a three-month suspension of the tournament, came back and finished the final round, and I had my first victory here".
Three down to Casey at day's start, Mickelson will go to sleep on a three-shot lead.
"I didn't make anything".
"I'm surprised we got this far [to the 16th green]", Casey said.
"Phil's put together a spectacular round of golf, 6 under, no dropped shots".
Scott Stallings joined Casey at 15 under, but Stallings can't make up any more ground. Along with an outside chance at forcing a playoff, finishing alone in second instead of a tie is a difference of $152,000, along with world ranking points and FedEx Cup points.
With a par at the 11th, Mickelson took a one-shot lead.
Mickelson was standing on the 17th tee when he heard the horn sound to stop play, and he shook his head. It was known as "Crosby weather" when Bing Crosby was the tournament host for the better part of a half-century.
Mickelson is on the verge of winning for the 44th time in his career, and matching Mark O'Meara with five victories at a tournament he first played in 1995.
Some playing groups had fun with the inclement weather though. They were waiting to tee off when sunshine gave way to a hail storm that pounded the golf course as players ducked under umbrellas.
Workers used squeegees and leaf blowers to remove the hail.
Sam Saunders, whose grandfather Arnold Palmer was among the Pebble Beach owners, scooped up hail and tossed it like a snowball.
Golfers, caddies and spectators raced for cover Sunday when hail began to pelt the course at the AT&T Pebble Beach Pro-Am.
Casey has never won in three previous times he had a 54-hole lead on the PGA Tour, all of them by two shots or more, and he was holding his own against the relentless pursuit of Mickelson, who missed three straight birdie putts from the 12-foot range by the slimmest margins. But I totally get where he's coming from. "I can see fine, and I'm playing well, so I wanted to continue and that's all there is to it. In all honesty, it's a good thing to play the last two holes in fresh conditions". Scott Piercy had a 15-foot putt that was slightly uphill, and he still ran it 7 feet by the hole and three-putted for bogey.
Australian Jason Day and Si Woo Kim, of South Korea, were in the clubhouse on 13 under.