Victory for Law would have equaled the LPGA record for a final-round comebackLPGA Mediheal Championship final-round leaderboard-7 SY Kim (Kor), J Lee (Kor), B Law (Eng); -5 C Hull (Eng), L Thompson (USA), EH Ji (Kor), A Yang (Kor) Full leaderboard
The English Bronte Law was behind 10 shots before he lost in a play-off like South Korean Sei Young Kim won the Mediheal championship.
The law hit the lead in California after a brilliant bogey-free, seven-under-par 65 saw her rise from the 20th.
The 24-year-old old from Stockport chose three consecutive birdies from the 10th for an eagle in the par-five 15.
Kim ended up with a triple 75, but won the play-off after bogeys by countryman Jeongeun Lee and Law.
The law probably goes from the seventh to the top five of the European Solheim Cup world ranking.
"My wave is now in a good place," she said. "If someone had told me that I would be in a play-off at the start of the day, I would have told them they were crazy."
Colleague English golfer Charley Hull, who was looking for her second LPA Tour title, finished fourth – two shots off the pace – after shooting a 74.