Mickelson tied the tournament's course record Thursday, shooting a nine-under-par 63 to take a one-shot lead over Gavin Coles in the first round of the final tuneup for Augusta.
Mickelson, the 2004 Masters champion, birdied six of the first seven holes.
"It was a fun day, a fun start," Mickelson said. "I birdied the first couple of holes and was off and running.
"I looked up and I was six-under through seven. That was a good feeling."
Coles' career-best 64 set the morning pace.
Charles Warren is third at seven-under 65, followed by Joe Durant, Ian Poulter and Scott Parel at 66.
Parel made a 21-foot birdie putt on No. 18.
Mickelson, the defending champion, missed a six-foot birdie putt on the 18th green that would have set a tournament record at the TPC at Sugarloaf in this Atlanta suburb.
There have been three other 63s since the tournament moved to Sugarloaf in 1997: Tiger Woods, 1998; Duffy Waldorf, 1999; and Ben Crane, 2003.
In 1979, when the tournament was held at the Atlanta Country Club, Andy Bean shot a record 61.