Retief Goosen, who collected his second U.S. Open crown last year at Shinnecock Hills, carded a 2-under 68 and is tied for third place with England's Lee Westwood and Brandt Jobe, who held sole possession of the lead at 4-under, but bogeyed 16 and 17.
Mickelson was 2-under par, but dropped shots at both 15 and 16. At the closing hole, where Stewart sank the 15-footer for par to win six years ago, Mickelson ran home a 20-foot birdie putt to join the group at 1-under par.
'I am pleased with 1 under,' said Mickelson, the 2004 Masters winner. 'I'll take even par or better anytime in U.S. Open. I think we're in for a tough three more days.'
Tiger Woods, a two-time U.S. Open winner who tied for third six years ago, scrambled to an even-par 70. He only hit six fairways on Thursday, but managed to get into a tie for 10th.
'Anytime you shoot even par in the Open, you ought to feel pretty good,' said Woods, the reigning Masters champion. 'And especially this one. I need to be more in the fairway. But I just kept hanging in there and kept grinding.'
Woods' problems with the driver were evident right away as he landed on a cart path at the par-5 10th, his first on Thursday. He hit a fairway-metal with his second and was able to reach the fringe. Woods chipped to 12 feet and sank the birdie putt.
He ran into trouble midway through his front nine. At the par-3 15th, Woods dumped his tee ball in the front bunker and made bogey. One hole later, Woods missed the fairway and was angered when photographers snapped photos during his backswing. His ball landed in another bunker and he was unable to save par.
The No. 1 ranked player in the world birdied the par-5 fourth to get back to even-par on his round.
But Woods and the rest of the golf world is looking up at Browne and Mediate.
Browne, who played in the second group out Thursday morning, did not get off to an ideal start. He bogeyed the second hole, but rebounded with a 10-foot birdie putt at the fourth. Browne bogeyed No. 6, but closed out his front nine with a tap-in birdie at the par-3 ninth.
The 46-year-old squandered makeable birdie chances at 10, 11 and 12, but rolled in a pair of 20-footers for birdie at 13 and 14. Browne collected his third birdie in a four-hole span when his 18-foot birdie putt at 16 found the bottom of the cup.
'I kept my ball where I needed to today,' said Browne, a two-time winner on the PGA Tour. 'When I missed a shot, I missed it in the right place. It was a very comfortable day for me.'
It was almost a day that Browne did not see. He came close to withdrawing from his sectional qualifier after shooting a 73 in his first 18 of the 36-hole qualifier. He came back to fire 59 and is keeping the momentum from two weeks ago.
'I was probably five minutes from not being here,' admitted Browne, who tied for fifth at the 1997 U.S. Open. 'I decided to play nine and see how it went and I managed to shoot 30 on that nine, and ended up shooting 59. It was a fun day and I'm glad to be here and it's continuing.'
Mediate, who finished fourth at Southern Hills in 2001, drained a 4-foot birdie putt at the fourth, but his round did not take off until the back nine. He reached the green in two with a 3-wood at the par-5 10th, then converted the 15-footer for eagle to reach 3 under for the championship.
'I didn't think I could really reach,' admitted Mediate. 'It was real firm up front and it just carried. I walked up there and it was right in the middle of the green. I'm trying to lag and make a four, and it went right in the center.'
He dropped a shot at the 14th when his approach flew the green down a hill. Mediate hit a 6-iron to 7 feet at the 16th to set up birdie and match Browne in the lead.
'I felt good right away,' said Mediate, who has battled back problems the last year. 'I was coming in hoping to see a little better play. I was looking forward to it.'
Vijay Singh was seen favoring his knee late in his opening round on Thursday. He still managed to shoot an even-par 70 and is tied for 10th with Woods, Adam Scott, David Toms, Tommy Armour III, Toru Taniguchi and Bob Estes.
Ernie Els, a two-time winner, posted a 1-over 71 and is part of a logjam in 17th. Among the notable players tied with the South African are: Fred Couples, Adam Scott, last week's Booz Allen Classic winner Sergio Garcia, 2003 champion Jim Furyk, first-round co-leader in 2004 Angel Cabrera and Chris DiMarco, who lost in playoffs at the last two majors.