The 17-year-old native of Smiths Falls, Ont., closed with a 3-under 69 for a two-stroke victory, finishing at 10-under 206 at Blackthorn.
"It's amazing to win and I'm so thankful that I got the sponsor's exemption this week," Henderson said. "I'm very grateful for the win here and I think it will be a big confidence booster moving forward."
After the victory, LPGA Tour Commissioner Mike Whan accepted Henderson's waiver petition for Symetra Tour membership before the age of 18. Last year, Whan denied Henderson's LPGA Tour age petition, preventing her from playing Q-school. She will be 18 on Sept. 10.
Henderson earned $22,500 for the victory, though that money and her previous earnings as a non-member will not count on the money list.
Henderson tied for fifth last week in the Women's PGA Championship, the second of the LPGA Tour's five majors.
"I've been busy, but I wouldn't change it for the world, it has been a lot of fun," Henderson said. "People often ask me how busy it is and why I don't take a week off, but I love being out here and I think every week I am learning a bit more about myself and my game and I think it is helping me be better."
Second-round leader Dottie Ardina of the Philippines was second. She birdied the final hole for a 74.
Henderson made three straight birdies on Nos. 11-13, parred the next four holes and bogeyed the last. She opened with rounds of 72 and 65.
"I was 1 under on the front nine and I knew I was near the lead, but I was kicking myself because I knew I needed to get it going," Henderson. "Holes 11 and 12 were huge turning points, where Dottie and I kind of switched positions, and then I was able to keep it from there."
American Selanee Henderson, Jessica Wallace of Langley, B.C., and Italy's Giulia Molinaro tied for third at 7 under. Selanee Henderson had a 65, and Wallace and Molinaro shot 70.