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.
Casey resumed with a three-foot par putt at the 16th hole on Monday, while Mickelson had already completed the hole.
Mickelson sits at -18 for the tournament, with three holes to play.
Mickelson recorded his 44th PGA Tour triumph in style with a closing seven-under-par 65 at Pebble Beach. "We had a chance to come out today on fresh greens, better weather and be able to see a lot better".
"Here I'm trying to hit the ball as far as I can, not anxious too much about the rough, balls plugging and we're able to clean it". Play was suspended on Sunday due to weather and darkness.
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.
"It's been a treat this week", said Casey, who had entered this week with two weeks of rest after a tie for second at the Singapore Open on the Japan Tour.
Maurizio Sarri delivers ultimatum to Chelsea board amid sack fears
I do my best and the players do their best. "We had only to stay in the match but the reaction was not good". At the moment we have a big problem away [from home].
As Jim Nantz stated in a post I wrote earlier this year, Pebble Beach seems like the ideal place for Lefty to finally get that U.S. Open win.
Phil Mickelson poses with the trophy after winning the Pebble Beach Pro-Am on Monday. And in 1992 he made his first start as a professional at the Pebble Beach Pro-Am.
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.
It's the course where his grandfather worked as a caddie and where he began his pro career. I'm hitting drivers and trying to bomb it down there and hit wedges in, and the rough will be so different and the greens will be so firm it will be a whole different golf course.
Mickelson needs to win the US Open to complete the Major Grand Slam and with Father Time accelerating along, playing this year at a venue that he knows so well and that he plays so well, offers him a chance.
Asked in a greenside interview what the performance did for his confidence, Mickelson said: "Absolutely nothing, because it's nothing like the course we'll see".
Mickelson, however, doesn't think the win this week will give him an edge in June. "But I'll deal with that in [four] months".