Ernst, who is in her second LPGA Tour season, came from two shots off the pace with a round of 67 to move past overnight leaders, Suzanne Pettersen and Carlota Ciganda.
Pettersen was the big shock of the last round, especially given she had won this event in two of her last three attempts. The Norwegian could only manage a final round of 74 and slipped from a share of 1st to 16th.
Chella Choi and So Yeon Ryu made it three Koreans in the first four when they tied for 3rd, two shots from the playoff.
Sarah Jane Smith and Lindsey Wright were the only two Australians to make the cut but they finished well back.