She played the last two rounds without a blemish on her scorecard, including a 10-under 62 in Friday's third round, and delivered 16 birdies in her final 36 holes.
The American, who earned $300,000 for the win, said her game clicked into gear unexpectedly on Friday.
"On Friday morning I wasn't playing too well and then I just found something and was able to turn it on and I just kept it going," she told the LPGA's official website.
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"I can't remember two and a half days after I struggled with my swing feel, and then made so many birdies in my life. It was an unbelievable run."
The 39-year-old credited an encounter with a lucky turtle for turning things around.
"I didn't even think about it until after I shot 10-under (on Friday) that turtles are really good luck for me. They always have been. I went swimming with the turtles on Friday afternoon and I inadvertently touched one," she added.
"The patrols are out there so that you don't harm them, but it kind of swam at me and I touched it, and it was really, really good luck."
Overnight leader Jang could have used some of that luck.
The sponsor's invitee had gone 59 holes without a bogey before dropping three strokes just before the turn to open the door for Kerr.
She finished with an even-par 72 and was one of only two players in the top 30 not to break par in the final round.
Jang, who usually plays on the Korean women's tour, ended in a three-way tie for second place with compatriot Chun In-gee (67) and New Zealand's world number one Lydia Ko (64) on 17 under par.
South Korea's Ryu So-yeon, who won the first major of the year at the ANA Inspiration earlier this month, finished alone in sixth place, five shots behind Kerr.
(Writing by Peter Rutherford; Editing by)
