The victory was his 34th on the PGA Tour.
Mickelson set up the winning putt with an approach shot from behind trees 150 yards from the hole.
"There were trees everywhere. But ... I thought I could hit a wedge through that gap on the green ... and it just came off perfectly," Mickelson said. " I never saw it. Once it took off, I never saw the flight of the ball because it was hidden from the trees, the overhanging trees in front of me. But I kind of ran out to the fairway and saw it end up ... that was a pretty cool feeling."
The birdie gave Mickelson a 2-under-par 68 and a total of 14-under 266 for a one-stroke win over Tim Clark and Rod Pampling.
Clark shot a 4-under 66 and Pampling a 68 in the final round, tying at 267. Stephen Ames was another two shots back at 269.
Pampling called Mickelson's wedge over trees to the final green "an amazing shot from Phil."
Mickelson won $1 million.