Mickelson finished with a 7-under 65 to win the tourney, matching the tournament record with his fifth victory.
Mickelson, who started the final round three shots behind, never came close to a bogey and built a three-shot lead through 16 holes Sunday night when it became too dark to finish, no matter how hard he lobbied to keep going.
Mickelson, seen shaking his head when the horn sounded Sunday night, said he thanked Casey on Monday morning for holding his ground because it was fair to both of them.
Casey finished with a birdie that was worth US$152,000 (NZ$225,000) because he wound up alone in second place.
That's what he had left at the 16th hole when darkness fell Sunday, suspending play.
This event differs from many on the PGA Tour, as the Pro-Am features 156 pro golfers and 156 amateurs.
He also won the Pro-Am with Don Colleran, the chief sales officer for FedEx.
Durban rescuers dig trench to save newborn stuck in a drain
When they finally got to the baby they broke into the pipe and managed to reach in and grab her. Medics said she may be suffering from hypothermia but otherwise appeared unhurt.
Mickelson won at Pebble Beach for the fifth time, matching the record set by Mark O'Meara. Dustin Johnson won the first of his two titles in 2009 when the final round was delayed until Monday because of rain, and then canceled because the storm system wouldn't leave.
Mickelson argued that he could "see just fine" on Sunday evening, moments after sunset with two holes remaining.
"That's one of the worst 3-footers I've had to hit", Casey said.
The weekend's PGA Tour event is still now ongoing and scheduled for a Monday finish due to adverse weather conditions over the weekend causing delays.
Mickelson's longest putt for par was 4 feet. "I just get in my own little bubble and I don't see the big picture". In second is Scott Stallings, who sits at -15 and has finished his tournament.
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. The US Open remains the final piece missing for him to complete the career Grand Slam, though Lefty was quick to caution that this week had no bearing on this summer.
After his 9-iron into a foot at No. 9 to get within one shot, Mickelson holed a 12-foot birdie on the 10th with a drive that hugged the right side of the fairway and likely would have bounced into the ocean if not for conditions so soft from rain that balls plugged where they landed. A missed cut in Phoenix, Mickelson said, was due in part to a driver experimentation that he quickly abandoned. I would love nothing more than to add to it five months from now, but that's so far down the road.