'It was certainly unexpected. I mean, I didn't hit it great today and somehow I shot 59,' Mickelson said. 'So go figure.'
Mickelson finished the event at 17-under-par 127, five strokes clear of PGA champion Vijay Singh. Retief Goosen, who won his second U.S. Open title in June, took third place at 11-under-par 133 while British Open winner Todd Hamilton finished fourth at 1-over-par 145.
'It was incredible,' said Singh. 'After about the 12th, 15th hole, we were just watching him.'
The memorable round began with a par on the opening hole at Poipu Bay Golf Course, but Mickelson picked up his first birdie of the day at the par-5 second to start an impressive run. He then rolled in a long birdie putt at the par-3 third.
Mickelson missed the fairway off the tee on the par-4 fourth on a day where his drives were quite erratic. Everything else in his game was working, however, and he hit his second shot to seven feet for his third straight birdie.
'Today I didn't really feel that sharp off the tee. It was when I got on the surface, when I got on the green, boy, I was just able to see the line and get the ball to go in,' said Mickelson. 'It's a good feeling.'
The 34-year-old landed his approach inside 6 feet for a birdie at the par-4 fifth and played his second shot to 14 feet at the par-5 sixth. Mickelson continued his putting display and ran the ball in the middle of the hole for an eagle.
Mickelson, who was three back to start the day, hit his second shot to 8 feet at the seventh and converted the birdie try to take sole possession of first. He brushed off another missed fairway at the par-4 ninth and knocked his approach inside 14 feet for another birdie to go out in 28, a Grand Slam record.
The barrage carried over onto the back nine and Mickelson hit his second shot to 6 feet at the par-4 10th. He missed the birdie putt, but countered at the par-4 12th when he almost holed out from the fairway for an eagle.
Mickelson sank the short birdie putt coming back and hit another drive in the rough at the following hole. He left his second shot outside of 30 feet, but calmly ran home the birdie try to move to 14 under.
At the par-5 14th, Mickelson missed the green with his second shot, but played his third within 8 feet of the hole. Once on the green it was business as usual and Mickelson drilled the putt for his third straight birdie.
The reigning Masters champion drained a 22-foot putt for a birdie at the par-4 16th, but missed the green at the par-3 17th. Mickelson got up and down for par to head to the par-5 18th needing a birdie to shoot 59.
'I knew that 18 was a birdie hole, but I had to hit the fairway to really have a good shot at it,' he said.
Mickelson, who eagled the 18th in Round 1, hit a perfect drive and knocked his second shot 10 feet short of the hole. The left-hander had a chance to shoot 58, but two-putted for birdie to complete the historic round.
'I don't have any aspirations of really ever going out and shooting 59. It just seems to happen,' said Mickelson, who joined a prestigious group that features Annika Sorenstam, David Duval, Chip Beck and Al Geiberger. 'What I really want to do is prepare myself for next year, and know that 59 isn't the goal, it's to start winning tournaments. When you start playing well, things like this happen.'
Singh saw his chance at winning the event vanish on the front nine as Mickelson made his way into the record books. Singh collected four birdies over his first nine holes to make the turn at 10 under before finding trouble with a bogey at the 11th.
The top-ranked player in the world missed the green en route to a bogey at the 11th, but recovered with a short birdie putt at the 11th. Singh then bogeyed the 13th, but again countered with a birdie at the very next hole. He added a pair of birdies down the stretch for a round of 66.
Goosen held the lead after the opening round and hit a 4-iron to 12 feet for an eagle at the par-5 second. The South African added a birdie at the fourth, but cooled down from that point on with a lone birdie at the 13th on his way to a 68.
Hamilton struggled to a round of 75 to finish well off the pace.
'I actually felt like I was in everyone's way today. I've been struggling with my golf for a while,' said Hamilton. 'But these guys all played well, and Phil's round was spectacular. I don't think I've ever seen a guy hole so many putts.'