Mickelson shot a 1-under 70 in the second round to finish 36 holes at 6-under 136. Pat Perez was one back after firing a 65, while those at 138 included Jimmy Walker, Matt Kuchar, Carl Pettersson, Jarrod Lyle and Jonathan Byrd.
After winning last week at Pebble Beach with a closing round 64, Mickelson has a chance to move into ninth place by himself on the all-time PGA Tour victory list. He is currently tied with Cary Middlecoff for ninth place with 40 wins.
Mickelson began his round Friday on the back nine and moved under par for the first time at the short 16th, where he holed a 35-foot chip shot for birdie.
He came to the par-4 eighth, his 17th hole, with an even-par round working. He then pulled off the shot of the day by holing his second shot from 110 yards out for an eagle.
Mickelson, however, missed putts inside six feet at the seventh, ninth and 18th holes -- suffering a bogey on all of them.
World No. 1 Luke Donald turned in a 1-over 72 and was at even par for the tournament in a tie for 28th place.
Swim Week Miami Beach 2014 [PHOTOS]