He started the day three shots off Casey's lead but roared to the top of the leaderboard with six birdies that put him 18-under for the tournament with two holes remaining.
Mickelson is now top of the leaderboard on 18-under with two holes left to play, while Casey is on 15-under with three holes left.
"I've written down a goal of winning L.A. and I had an outside chance of winning this one - I had a very good chance of winning this one", Casey said.
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 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.
American Scott Stallings was in the clubhouse on 15-under after a six-under par 66 with Jason Day of Australian and South Korean Kim Si-woo tied for fourth on 13-under.
"I was coming here this week with a little trepidation", said McDowell, who does not get to join McIlroy or Seamus Power in Los Angeles for this week's Genesis Open and will be playing in opposite field Puerto Rico Open next week rather than the $2.25 million WGC Mexico Championship. Casey and FedEx executive Don Colleran had a one-shot lead in the pro-am. "I'll deal with that in six months".
This was a fantastic win for the 27-year-old, a protege of fellow Aberdonian and former Open champion Paul Lawrie.
@PhilMickelson is a 44-time victor on the PGA TOUR. He was on even par for the round when play was suspended.
"I'm looking forward to getting back at it at Bay Hill", Day said. He preferred to suspend play with his ball on the 16th green, resuming the tournament on Monday when the greens, which took a beating from rain Saturday night and an impressive hailstorm Sunday, would be fresh. A birdie at the second was followed by eight straight pars before back-to-back bogeys at 11 and 12.
Mickelson, however, doesn't think the win this week will give him an edge in June.
"I played very well around here on Thursday in good conditions and only shot 3 under", Casey said.
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