The 20-year-old from Burton Latimer started the final round with a one-shot lead over American Brittany Lincicome and South Korea's Ryu So-yeon, and produced a brilliant, bogey-free 66 to finish two clear on 19 under par at Tiburon Golf Club.
"It's a pretty cool feeling," said Hull. "It hasn't quite sunk in yet.
"I just feel pretty good where I am at the moment. I feel like I'm mentally good. I feel relaxed.
"I think that's the key for me - if I come into a week relaxed, I feel good. This week it happened for me. I'm confident; I'm happy.
"It feels great to be a winner on the LPGA Tour. It's wicked to do at 20. It's fun."
Hull had six birdies on the outward nine and three on the back nine en route to a new tournament scoring record, previously set in 2015 by Cristie Kerr at 17 under.
Hull and Ryu were locked together on 18 under with two holes to play, but a two-shot swing at the par-five 17th - Hull rolled her approach up for birdie while Ryu found a greenside bunker and could only manage a bogey - turned proceedings decisively in the Briton's favour.
With her only previous professional win having come on the Ladies European Tour in 2014, Solheim Cup star Hull admitted she was glad to have ended a lengthy spell without a victory.
"I last won on the LET, the European Tour, like two years ago," she added. "I had that dry patch for a few years. Feels good now to get a win.
"It was a lot of pressure coming from when I played in the Solheim Cup. I was 17 years old and I had five second-place finishes in my first five starts, and took me over a year to win on the LET.
"Now this is actually my first full year on the LPGA Tour. Last year I didn't have full status. To win, it feels good. Feels like I got the chip off my shoulder."