This was his fifth win at Pebble Beach, a handsome golf course that he clearly knows very well and is also staging the US Open this year.
The popular lefthander has been a US Open regular since Irwin's victory nearly 29 years ago and he's finished second an incredible six times in the event.
He also won the Pro-Am with Don Colleran, the chief sales officer for FedEx. I would love nothing more than to add to it five months from now, but that's so far down the road.
'Sometimes I get in my own bubble, ' Mickelson said.
Mickelson was standing on the 17th tee when he heard the horn sound to stop play, and he shook his head.
The victory, Mickelson's first since the WGC-Mexico Championship last March and first on USA soil in six years, carries special significance for him.
Casey was pleased to join Colleran as a Pro-Am champion after a near miss previous year.
He was at his best at Pebble Beach, and no one had a chance.
Expanding on why he felt his performance at Pebble Beach boded well for Augusta, the three-time Masters victor said: "The two areas that you have to be at your best to play well at Augusta are you've got to hit the ball long and have a really good short game and those were two areas that were very helpful here".
Despite having led overnight, I don't think Casey blew it - the American's 65 would have seen off most people - and he can be happy with the good start that he has made to the year.
"That's probably a good omen", Mickelson said. "This golf course, it looks very gettable [but] it can expose you". And it is where he will turn 49 on the final day of the 2019 U.S. Open. A awful tee-shot was followed by a chip that failed to find the green and a third that wasn't much better and after a fast start, which had seen him birdie the first two holes and three of the first five, and following a run of 11 straight pars, the 38-year-old Aussie missed his bogey attempt and tapped in for a double-bogey five.
He gained ground with birdies at the par-five second and the fourth, then made back-to-back birdies at the ninth and 10th - two of the toughest holes on the course.