Korda, the winner last year, had a 72 after taking a one-stroke lead over Kupcho into the final round. Korda dropped out of the playoff with a three-putt par on first extra trip down the par-5 18th.
Maguire got new life when Kupcho's 2-footer slid by the left side.
“I missed the putt,” Kupcho said. “I almost missed the putt in regulation, the same putt. That comes with a lot of nerves and a lot of shaking.”
Kupcho won for the second time on the LPGA Tour, following her major victory in early April at Mission Hills in the California desert. The 25-year-old former Wake Forest star from Colorado won the inaugural Augusta National Women’s Amateur in 2019.
Kupcho got off to a slow start with a double bogey on the par-4 third and a bogey on the par-4 sixth. She rebounded with an eagle on the par-5 eighth and birdie on the par-4 ninth. On the back nine, she birdied the par-4 12th and par-5 14th and bogeyed the par-4 16th.
“I struggled a little bit at the beginning, so it tells me I can really come back from it,” Kupcho said. “I’ve always known I’m a back-nine player and that definitely came into effect today.”
Maguire left the course without commenting to media. She won the LPGA Drive On Championship at Crown Colony in early February to become the first Irish winner in LPGA Tour history.
The second-ranked Korda made her second start since returning from a four-month break because of a blood clot in her left arm. She tied for eighth two weeks ago in the U.S. Women’s Open in her first event since early February.
“Unfortunately, sometimes you have it and sometimes you don’t,” Korda said. “If you told me I think three, four months ago when I was in the ER that I would be here I would being extremely happy.”
Korda also is the defending champion next week in the KPGA Women’s PGA at Congressional.
Lydia Ko was fourth at 17 under after a 68.
Lexi Thompson, the 2015 winner, closed with a 70 to tie for fifth at 16 under with Jessica Korda (67), Nelly's older sister, and Carlota Ciganda (67) and Atthaya Thitikul (68).
“Of course, it’s not the outcome that I wanted, but I felt strong the whole week and played great golf, and nothing but positives to take into a major next week,” Thompson said.
Brooke Henderson, the ShopRite LPGA Classic winner last week in New Jersey and 2017 and 2019 champion at Blythefield, had a 72 to finish at 15 under,
Minjee Lee, the U.S. Women’s Open winner, was 11 under after a 74.