The 29-year-old Scot beat Kana Nagai by four strokes for her first LPGA Tour title, while overnight leader Momoko Ueda tied for fifth at 12-under 276.
The two-time winner of the tournament birdied the final hole of the third round to remain on top. Gemma Dryburgh is one shot off the pace.
The 2007 and 2011 tournament winner has a one-stroke advantage over compatriots Ai Suzuki and Miyuu Yamashita.
Ai Suzuki, the 2019 tournament champion, and Momoko Ueda carded 7-under 65s to finish one stroke ahead of compatriots Ayaka Furue and Sakura Koiwai.