The start of the day's play was delayed by an hour for rain and stopped again for two hours as a hailstorm hit the California course. That was start of a five-hole stretch when Mickelson made three birdies and Paul Casey had two bogeys.
"I get exactly where Paul is coming from", Mickelson later said. The 48-year-old Mickelson said, "I have pretty good vision".
It is Mickelson and Casey now battling to win the tournament, while Scott Stallings also has an outside chance.
He has gone 28 years since his first PGA Tour victory and his 44th, and he joined Tiger Woods as the only players to surpass $90 million in career earnings. "I've got two hard holes coming up".
Mickelson hit 7-iron to 8 feet on the par-3 17th and played it safe down the par-5 18th, making a 6-foot birdie for a three-shot victory.
"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. "Sometimes I get in my little bubble and don't see the bigger picture".
Casey simply couldn't see, and we realized it was not possible to finish all 18 holes - "We can't finish two holes in six minutes", he said to the official - he opted to mark his ball on the 16th green.
Phil Mickelson celebrates his victory at the AT&T Pebble Beach Pro-Am.
"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".
Casey was pleased to join Colleran as a Pro-Am champion after a near miss a year ago. Play was suspended on February 2 and resumed and ended on August 17. Casey said there was no way to finish and they had to return Monday morning.
Casey called Mickelson's effort "a phenomenal round of golf" but added: "It's not over yet".
Casey has never won in three previous times he had a 54-hole lead on the PGA Tour, all of them by two shots or more, and he was holding his own against the relentless pursuit of Mickelson, who missed three straight birdie putts from the 12-foot range by the slimmest margins.
With a total of five bedrooms and seven baths, four bedroom suites open to ocean-view terraces, including the master suite that includes a sitting room, glamorous marble bath and closets.
American Scott Stallings holds the clubhouse lead on 15 under after his six-under par 66, with Jason Day finished on 13 under, level with South Korea's Kim Si-woo.
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