Casey could barely see the 3 feet of grass between his ball and the cup on the 16th hole and said there was no way to complete two holes in six minutes.
"I take a lot from this", Casey said. "Phil has put together a spectacular round of golf so far - 6 under, no dropped shots".
With the light failing on Sunday, Casey opted against trying to get the tournament finished.
He also won the Pro-Am with Don Colleran, the chief sales officer for FedEx. "Let's play 17 and see how it goes".
It won't be the last time Mickelson gets to play Pebble Beach this year.
The victory, Mickelson's first since the WGC-Mexico Championship last March and first on US soil in six years, carries special significance for him.
This was a fantastic win for the 27-year-old, a protege of fellow Aberdonian and former Open champion Paul Lawrie.
Sam Saunders, whose grandfather Arnold Palmer was among the Pebble Beach owners, scooped up hail and tossed it like a snowball. Patrick Reed's brother laid on his back and tried to make a snow angel. "This golf course, it looks very gettable [but] it can expose you".
Mickelson claimed his 44th US PGA Tour title and has also won three European Tour titles, the 2004 Grand Slam of Golf and 2004 Skins Game, and a 1993 Challenge Tour event in Paris.
This was Mickelson's fifth AT&T title so it's fair to say he was a tournament specialist before this week but he was still generally a [36.0] chance and he even hit a high of [50.0] before the off. Casey, who traded at a similar price to Lefty pre-event, hit a high of [80.0] and a low of [1.5] in-play. Mickelson's first goal is that fourth green jacket and he believes that even now, at the age of 48, he has the power and the putting touch to give McIlroy, Jordan Spieth and the rest a run for their money in eight weeks' time.
He just didn't want to stay another day.
"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". "It's dark and we're going to have a good chance tomorrow to come out on fresh greens".
"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". He birdied five of the first seven holes in a bogey-free round.
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.