By Mark Lamport-Stokes
The American left-hander, a double champion at Augusta National, watched as his ball pitched six feet beyond the hole before rolling back down the slope into the cup to move him to 11-under.
He raised his arms skywards in disbelief as the crowd packed around the green roared its approval.
Mickelson had eagled the par-five 13th after hitting his second shot to six feet and calmly knocking in the putt.
Fellow Americans Dan Pohl, in the third round of the 1982 Masters, and Dustin Johnson, in the fourth round last year, had previously posted back-to-back eagles at Augusta.
(Editing by John Mehaffey)