All four players return to Pebble along with numerous game's best, including Jordan Spieth, Tony Finau and Jason Day.
A Monday finish brought on by rain and a freak hailstorm was only a minor inconvenience for the 48-year-old Mickelson.
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.
For more sports coverage, visit the ABS-CBN Sports website. They got pretty rough this afternoon so I totally get it."But I have pretty good vision, I can see fine and I'm playing well so I wanted to continue, that's all there is to it".
Wade Ormsby, a pre-tournament [80.0] chance, hit a low of [1.18] during the fourth and final round of the ISPS Handa Vic Open on the European Tour yesterday but long odds-on players got their fingers well and truly burnt.
"It's been a lot of work - bio-mechanic swing studies, taking weaknesses and making them strengths, time in the gym - but days like this make it worthwhile".
"And so it's gratifying to see the results and to finish it off the way I did".More news: USA warships again challenge Beijing's claims in South China Sea
"We've got two tough holes left and I don't want to jump the gun and get past that", Mickelson said.
Hale Irwin, who won the 1990 edition of the US Open at 45 years and 15 days old, is the oldest US Open victor to date so the stats are really against Phil Mickelson, who turns 49 on the Sunday of this year's edition.
There was plenty of sunlight for the Monday morning finish, just no drama. The greens were beat up (on Sunday).
After playing the 17th and 18th on Monday, Mickelson admitted Casey was right.
Mickelson was standing on the 17th tee when he heard the horn sound to stop play, and he shook his head.
"I thank Paul for having us come back out here this morning", Mickelson said.
Mickelson finished at 19-under 268.
He also won the Pro-Am with Don Colleran, the chief sales officer for FedEx. And it is where he will turn 49 on the final day of the 2019 U.S. Open.More news: Trump's racist Elizabeth Warren taunts have entered a new phase
He also became the oldest-ever PGA victor at the famed California coastal layout. The other three times he was 2 over par or worse in the final round.
Casey called Mickelson's effort "a phenomenal round of golf" but added: "It's not over yet".
Mickelson made his big run starting with a 9-iron to a foot behind the cup on the par-4 ninth. The US Open remains the final piece missing for him to complete the career grand lam, though Lefty was quick to caution that this week had no bearing on this summer. He finished three shots behind Graeme McDowell in 2010, the last US Open at Pebble.
Pebble Beach was so soft that balls were plugging in the fairway when they landed.
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.
Mickelson lives for the moment.
Mickelson now has a runner-up finish at the Desert Classic and a victory in four starts this season.
It's the thrill of competition.More news: Newborn with umbilical cord still attached found in middle of road