John Mallinger and Nick Watney also shot rounds of 7-under-par 65 on Thursday to join the reigning Masters champion atop the leaderboard.
Jim Furyk, the No. 2 ranked golfer in the world, Arjun Atwal and Jason Schultz are knotted in fourth place at 5-under-par 67.
Mickelson and Watney played the easiest of the three courses, Poppy Hills, while Mallinger teed it up at Pebble Beach. Poppy Hills is the most protected of the three courses and that was important because Thursday was a wet and windy day.
Mickelson began on the back nine Thursday and parred his first two holes. He rattled off three consecutive birdies from the par-5 12th, then parred No. 15.
Mickelson, a three-time major winner, recorded back-to-back birdies at 16 and 17 to make the turn at 5-under-par 31. On his second nine, Mickelson found himself in the teeth of the demanding weather conditions.
The lefthander had one trick left up his sleeve. He parred the first eight holes of his second nine, then hit a spectacular 4-iron approach at the par-5 ninth to set up eagle and get him into a share of the lead.
'It was a fun day, even though it was windy at times, and I don't think I've ever driven the ball better before,' said Mickelson, who won this title in 1998 and 2005. 'The greens were in great shape today and the key was that I started every putt on line at the right speed.'
Mickelson has had an erratic start to the 2007 season. He tied for 45th at the Bob Hope Chrysler Classic, his first action since the Ryder Cup in September, then shared 51st at the Buick Invitational.
Last week, he missed the cut at the FBR Open, but Mickelson insists he's improving.
'My game has been progressing from week to week and I really can't wait to test my driving in tough conditions.'
Mallinger endured the most trying conditions of the three leaders. He teed off on No. 10 and flew out of the gate with a 40-foot birdie putt at 10. Mallinger, a TOUR rookie, rolled in a pair of birdie putts at 11 and 12, then tapped in a short birdie putt at the par-5 14th.
He closed his front nine with a short birdie putt at the 18th, then took advantage of the next par-5, the second. Mallinger two-putted for birdie there and collected his final birdie of the round at four when his sandwedge approach stopped 7 feet from the stick.
Mallinger parred out when the conditions got really demanding.
'It was perfect probably for at least nine holes,' said Mallinger. 'The wind started picking up right around three, so it was probably 13 holes I got that were pretty nice.'
Watney, like Mickelson, teed off on the back nine at Poppy Hills. He notched three birdies and an eagle on his opening nine, but dropped a stroke at the first.
Watney, whose first top-10 on the PGA TOUR came last year at Pebble Beach, recovered with birdies at the fourth and sixth holes to join Mickelson and Mallinger in first.
Last week's FBR Open champion Aaron Baddeley, Dean Wilson, Ryan Armour, Brendon de Jonge, Greg Owen, Charles Warren, Craig Kanada, Kyle Thompson, Sony Open in Hawaii winner Paul Goydos and two former U.S. Open champions, Lee Janzen and Corey Pavin, share seventh place at 4-under-par 68.