Brittany Lang posted a 6-under-par 66 to stand second, while Soo Young Moon is one stroke further back at minus-5. Hee Jung Park stands alone in fourth place at 4-under-par 68.
After parring her next three holes, Ahn sank back-to-back 4-foot birdie putts from the 16th to help her make the turn at minus-3.
The 21-year-old birdied the first three holes of her back nine to jump to 6 under. Ahn rolled in putts of 10, 12 and 15 feet as she moved into a tie for the lead with Lang.
Ahn, who gained LPGA membership by winning the 2003 CJ Nine Bridges, moved ahead of Lang with a chip-in birdie on the par-5 eighth, her 17th, to grab the lead.
'When I woke up this morning it was very windy, so I thought it was going to be a cold day, so I had long pants on,' Ahn admitted through a translator. 'However, it ended up being a really hot day so it was kind of hard.
'There was a lot of wind, but it was helping me out today. Because of the wind, I had a hard time picking clubs. It ended up working out good, though.'
Lang, a former standout at Duke, also started on the back nine and got going with a birdie on No. 10. She sank a 7-footer for birdie at 15, then drained a 25-foot birdie effort on 16 to move to minus-3.
The 20-year-old two-putted for birdie on 17 to make it three in a row. Lang's drive at the second hit a spectator and she was left with a tough lie in the rough. That led to a bogey, dropping her back to minus-3.
Lang, who along with Morgan Pressel shared second place last year at the U.S. Women's Open when both were amateurs, came right back with a pair of chip-in birdies from the third.
She knocked her approach to 2 feet at the sixth and kicked that in for birdie. Lang missed the green at the par-3 seventh and was unable to get up and down for her par.
Lang got that stroke back with an 8-foot birdie putt on the eighth before a par on nine left her one shot behind Ahn.
'I played well today. I got off to a good start and had two chip-ins,' said Lang, one of four leaders after the opening round of last year's U.S. Women's Open. 'I made a lot of putts and chipped in twice so that helps. This is my second time leading, first as a pro.'
Defending Women's British Open champion Jeong Jang carded a 3-under-par 69 Thursday. She is joined in fifth place by Becky Iverson, Nicole Castrale, Rachel Hetherington, and Shinobu Moromizato.
Lorena Ochoa, the defending champion, leads a group of 13 players one stroke further back at minus-2.