Shin, who has recorded just two top-10s on the LPGA Tour this season, fired a flawless seven-under 66 to vault to the top of the leaderboard on a hot, humid day at Westchester Country Club.
Canadian teen Brooke Henderson, playing in the season's second major on a sponsor's exemption, also got to seven under but bogeyed her final hole, the par-five ninth, to open with a 67.
Australian veteran Karrie Webb, a seven-time major winner, was a further stroke back after carding a six-birdie 68, level with England's Charley Hull and Thailand's Moriya Jutanugarn.
"I was chasing the leaderboard all day and I was surprised that Webby was at six (under) at one point and I thought, how is she shooting six-under par," Shin told reporters after a bogey-free round that also included five birdies.
"I got there and I was like, 'Maybe I can get one more on the 18th hole?' And I did.
"I've been playing okay, and I feel like I have my first win coming up. It's a major, I want to play well."
Long-hitting Brittany Lincicome, winner of the year's first major, the ANA Inspiration, had set the pace with 10 others earlier in the day by shooting a 70 in the morning wave.
Lincicome said she had hoped to be able to take advantage of the five par-fives with her length off the tee on the classic, tree-lined course that has hosted more than 40 PGA Tour events.
"See my eyes light up?" the 29-year-old American told reporters with a smile. "Super excited."
Seventy was also posted by twice major winner Cristie Kerr, world number four Kim Hyo-joo of South Korea, 11th-ranked Lexi Thompson, Germany's Sandra Gal and Jessica Korda, who birdied 17 and 18 to join the party.
Two-time defending champion Park In-bee of South Korea launched her title defence with a 71 while New Zealand’s world number one Lydia Ko closed with birdies on the last two holes to register a 72.
"I was making some crucial putts and if they didn't go in, I know my score would have been much worse," said the 18-year-old Ko. "I think one-under is a pretty good start to the week. The scores are not going to be crazy low."
(Editing by Mark Lamport-Stokes/Steve Keating)
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