Mickelson had wanted to finish in the dark on Sunday and was left frustrated when Casey refused to play on - but a day later he understood the Englishman's stand. Casey had great par saves, and then Mickelson took off.
Mickelson was at his best on a course he loves.
For much of the final day, Ormsby's only serious danger had looked to be pre-event [130.0] shot, Brad Kennedy, and the veteran Sydneysider hit a low of [2.1] himself as the pair duelled it out on the back nine but it was all change late on when [1000.0] chance, David Law, rallied superbly after calling a penalty on himself on the ninth.
He finished at 19-under 268 and joined Tiger Woods as the only players to surpass US$90 million (NZ$133.7 million) in earnings.
Casey's consolation prize was teaming with Don Colleran to win the pro-am competition.
Casey, 41, is now 1 for 5 in trying to close out 54-hole leads on the PGA Tour.
Making it even more special is Pebble Beach, where he made his pro debut at the US Open in 1992, where he won the first of his five titles in another Monday finish in 1998, this one in August because of rain.
Mickelson recorded his 44th PGA Tour victory in style with a closing seven-under-par 65 at Pebble Beach.
"There's still a glimmer", said Casey said of his chances, who was level par for his round when play was suspended.
Mickelson and Casey were the only players who did not complete the weather-delayed event on Sunday night. "I just get in my own little bubble and I don't see the big picture".
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.
Phil Mickelson was not happy at having to return on Tuesday to play the final two holes after bad light stopped play.
It also would be his first victory on American soil since the Phoenix Open six years ago. He won that summer's British Open at Muirfield and last year's Mexico Championship. This is obviously the one major that Phil needs to win to lock up the career grand slam.
The four-time Pebble Beach victor nearly holed out from the ninth fairway to leave a tap-in birdie and complete a front-nine 33, before rolling in from 12 feet at the next to move into a share of the lead. Officials chose to complete play Monday, a decision that CBS's Gary McCord said had Mickelson "fuming". "I'll deal with that in six months".
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.