Mickelson, who could join Mark O'Meara as the only five-time winners of the tournament, is no stranger to the fickle February weather on the Monterey Peninsula.
"It has been a special week for me", Mickelson said.
There was plenty of sunlight for the Monday morning finish, just no drama.
Beginning the day three strokes off the pace, Mickelson matched Casey's birdie at the par-five second and holed a 10-footer at the fourth to cut the lead to two. Mickelson never came close to making bogey and won for the 44th time on the PGA Tour.
Casey was just shy of the green in two, but got up and down from six feet to finish second alone.
He also won the Pro-Am with Don Colleran, the chief sales officer for FedEx. 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.
"I've written down a goal of winning L.A. and I had an outside chance of winning this one - I had a very good chance of winning this one", Casey said.
Law stroked the eagle putt home to draw level before Ormsby made a complete hash of the par three 17th.
"A phenomenal round of golf, " Casey said.
Mickelson made his big run starting with a 9-iron to a foot behind the cup on the par-4 ninth.
Mickelson can thank the weather for having to come back to play on Monday morning. 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. The US Open remains the final piece of the career Grand Slam for Mickelson, who already holds the record with six runner-up finishes.
His first one also was a Monday finish, when players returned in August - more than six months later - to finish the third round of the weather-plagued tournament to make it official.
I think the Vic Open's unique format - men and women playing in alternate groups on the same course and for equal prize money - is great, although I can't help thinking: why not go further?
Mickelson lives for the moment.
It's the thrill of competition.
His path to victory these days is never going to be mundane, even in a tournament where he went an entire round without missing a fairway - a 20-year first.