Ariya (64) had a three-shot lead on the back nine but a bogey on 15 proved costly as the Thai was unable to hold off the charging Feng, who finished on 29-under 259.
Feng, whose previous win came in November 2017, hit a seven iron to three feet on the last hole to set up a crucial birdie putt.
“Before I putt, I looked up at the leaderboard and I’m like, ‘Oh no,’ I had to make that one,” Feng told reporters.
“So actually the last putt was under a lot of pressure. I was like, ‘You know what? You’ve been doing really well this week. Just make another good putt.’ That’s what I did.”
Feng entered the final round in a four-way tie for the lead alongside Ariya, South Korea’s Park Sung-hyun and American Tiffany Joh.
Park, who reclaimed the world’s top ranking with a win last week in Arkansas, carded a 69 to finish in a tie for sixth on 23-under, while Joh (67) tied with Amy Yang (65) for third at 25-under. Kim Hyo-joo (64) finished fifth at 24-under.
American Yealimi Noh, a 17-year-old Monday qualifier in her first LPGA event, tied with Park for sixth.
“It was my goal to be inside the top 10 for this week, and I’m really happy with how I finished,” Noh said.
Writing by Jahmal Corner in Los Angeles; Editing by Peter Rutherford