Mickelson carded a 2-under 69 and completed 54 holes at 13-under-par 200. He is one stroke clear of fellow second-round leader Padraig Harrington, who shot 70 in round three.
Former PGA champion Rich Beem had the shot of the day as he aced the par-3 14th at Riviera Country Club. That helped him card the low round of the day, a 6-under 65, which moved him from a tie for eighth to solo third.
'I stole my reaction from Peter Jacobsen. He made one I don't know how many years ago, but he hopped into the convertible and acted like he was driving,' said Beem, who jumped on top of a nearby Nissan that he won for the hole-in-one. 'I've been working really hard on my game. I've been swinging at it great so it is nice to see the work payoff.'
Beem stands at 11-under-par 202. He is searching for his first PGA TOUR win since the 2002 PGA Championship. Robert Allenby, the 2001 winner here, and Charles Howell III share fourth place minus-10.
Mickelson looked like he was going to run away from the field early. He converted back-to-back birdie chances from the first to pull two clear of Harrington.
The Irishman responded with a birdie of his own at the third. However, Harrington gave that stroke back when he was unable to get up and down for par after missing the green at the par-3 sixth.
Mickelson parred seven straight from the third, six of those were two-putt pars. He dropped his approach within 3 feet at the 10th to set up his third birdie of the round.
The 36-year-old Mickelson came right back with an up-and-down birdie on the par-5 11th. Harrington matched that birdie to remain three back.
Things took a turn for the worse for Mickelson. He knocked his second shot to the back fringe on 12 and three-putted for bogey from there.
Mickelson missed a 6-foot par putt at the next to slide back to 13 under.
After a pair of pars, he missed the green at the par-3 16th and again could not save par. That bogey dropped Mickelson into a share of the lead with Harrington at 12 under.
Mickelson steadied himself with a 4-foot birdie putt on 17. He nearly chipped in for birdie on 18, but settled for a tap-in par and a one-stroke lead.
'I fought my swing a little bit all day even though I was 4 under through 11,' said Mickelson. 'Those three bogeys let a lot of people back into it. A lot of people have a chance tomorrow. On the other hand, I am leading going into the final round, which is a good thing.'
Harrington, after his birdie on 12, parred the final seven holes to finish one back.
'I had a few chances there in the end, some 6-foot putts, that didn't go in, but I'm not too worried about that,' Harrington stated. 'It would have been nice to get a few birdies out there and get to 15 or 16 under par and take a few of the guys out of it. Of course, we still have to play the last day anyway.'
Former major championship winners Ernie Els and Jim Furyk each shot 67 on Saturday. They stand alongside Sergio Garcia (69) at 9-under-par 204. K.J. Choi and U.S. Open winner Geoff Ogilvy are one stroke further back at minus- eight.
Defending champion Rory Sabbatini is tied for 11th at 7-under-par 206.