He finished at 19-under 268 and joined Tiger Woods as the only players to surpass $90 million in earnings.
Casey finished with a birdie that was worth US$152,000 because he wound up alone in second place. He also won the pro-am with Don Colleran, the chief sales officer for FedEx.
Even so, it was the fourth time Casey took a 54-hole lead of at least two shots into the final round on the PGA Tour and failed to win. There wasn't much he could do to stop Mickelson, who at age 48 looks just as tough as when he won his first PGA Tour event in 1991 when he was still at Arizona State.
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.
Mickelson argued that he could "see just fine" on Sunday evening, moments after sunset with two holes remaining. Casey said there was no way to finish and they had to return Monday morning.
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.
"Sometimes I get in my own bubble," Mickelson said.
Scott Stallings finished Sunday night with a 66 to finish alone in third.
Mickelson won on American soil for the first time since the Phoenix Open in 2013. He won that summer's British Open at Muirfield and last year's Mexico Championship.
He will return to Pebble Beach in June for the U.S. Open, where he made his pro debut in 1992. The U.S. Open remains the final piece missing for him to complete the career Grand Slam, though Lefty was quick to caution that this week had no bearing on this summer.
Pebble Beach was so soft that balls were plugging in the fairway when they landed. And while the fairway lines already have been brought in to be much narrower than usual, the rough was light.
"It's nothing like the course we'll see," Mickelson said. "I'll deal with that in six months."
For now, he was glowing over another victory that keeps him as relevant as ever. Along with five titles at Pebble Beach, he ties Woods and Billy Casper — all three native Californians — with his 14th career victory in the Golden State.
- AP