American Ernst carded a five-under-par 67 in the final round to finish at 14 under overall as a play-off loomed at Columbia Edgewater Country Club with South Korean Kim firing a 68.
Only one hole of the play-off was needed after Kim bogeyed the par-four 18th, paving the way for 22-year-old Ernst to par and lift the trophy in Portland.
Ernst breathed a sigh of relief after surrendering a two-stroke lead on the back nine with consecutive bogeys from her final two holes.
South Korean pair Chella Choi (68) and Ryu So-yeon (70) finished two shots off the pace at 12 under.
Frenchwoman Karine Icher (66) was a further stroke back alongside South Koreans Choi Na-yeon (68) and Ji Eun-hee (70), and Dane Line Vedel (70).
At 10 under for the tournament was South Korean Lee Ilhee (68), who was joined by six others.
South African Paulo Reto (68), Americans Lizette Salas (69) and Laura Diaz (71), Colombian Mariajo Uribe (71), China's Lin Xi Yu (72) and South Korean Hur Mi-Jung (73) were also four shots adrift.