"Words can't even describe the feeling I have right now," said Thompson. "It's sort of like a rerun from Navistar, just having that feeling walking up to the 18th green, embracing the fans and just knowing you have the win under your belt.
"It's very big, I have worked so hard between this whole last year since Navistar, it pays off and means so much to me."
South Korea's Amy Yang was fifth, nine shots off the pace.
Sandra Gal of Germany, who shot a 5 under 66 in the final round, finished with a share of sixth alongside five other players: Chella Choi (-4), Alison Walshe (-2), Stacy Lewis (-2), Beatriz Recari (-2) and Karine Icher (+1).
Thompson began the day atop the leaderboard, leading nearest rival Lee of South Korea by three strokes, but the comfortable margin was down to two after the first nine as Lee made her move.
Lee picked up two back-to-back birdies on the 7th and 8th to close in.
"She (Lee) made a few birdies in that stretch of holes that I missed a few birdies putts. It got really close," Thompson said, who bogeyed the 2nd to drop a shot.
"It was hard to grasp because I actually hit some really good putts and they just lipped out unfortunately," she added.
Pressure on Thompson eased a little when Lee bogeyed the 9th.
On the back nine, Thompson came back strongly and fired three straight birdies to put her back in a comfortable position.
"Those were big. No. 10, I just hit a close wedge shot and made like a four-footer, but the birdie actually on 11 was really unexpected.
"But those are always huge in a tournament, getting into the next hole and always pumping yourself up to make more birdies out there."
She added another birdie on 16th to close in on the title she went on to clinch despite a bogey on the next.
