Defending champion Hannah Green held a share of the lead with six holes to play at the LPGA Tour's Portland Classic before she crashed out of contention with four bogeys.
The Australian went into the final round just two shots behind the leader and was three under after 12 holes to sit at 13 under.
But a dropped shot on the 13th and a trio of bogeys to close out left major winner Green signing for a one-over 73 and a nine-under 207 total for a share of 12th.
England's Georgia Hall missed a par putt on the 18th for the win in regulation but kept her composure to beat Ashleigh Buhai on the second hole of their playoff for her first LPGA Tour title in the United States.
The 24-year-old Hall, the 2018 Women's British Open champion, matched Buhai with a par on 18 on the first extra hole and won with a par on the par-4 1st at Columbia Edgewater.
"To bogey the last I was quite upset about that," Hall said.
"I had to refigure myself and get back to try and win that playoff."
Hall closed with a 4-under 68 to match South African Buhai at 12-under 204.
"I knew the scoring wouldn't be a low. I had a really good run in the middle there, three or four in a row. Had to hold on the last few holes," Hall said.
The 31-year-old Buhai birdied four of the last five holes for a 65 – matching Danielle Kang for best round of the day.
"Hit good shots coming down the stretch," Buhai said.
"You know, adrenaline. It's difficult to really calculate. Not much you can do, and just a misread putt. Just misread it a little bit."
She missed a chance for her first LPGA Tour victory.
Moriya Jutanugarn (67) and Yealimi Noh (69) finished a stroke out of the playoff.
Second-round leader Mel Reid (74) finished at 10 under with Inbee Park (66), Caroline Masson (66), Mariah Stackhouse (67), Cheyenne Knight (68), Robynn Ree (66) and Jasmine Suwannapura (68).
Green's fellow Australians Minjee Lees (69, three under) and Sarah Kemp (70, two under) finished in a share of 42nd and 51st respectively.