The 18-year-old Korean shot a 3-under 69 Sunday to win the Team WLF.org Golf Classic, making an eagle and three birdies at Kankakee Elks Country Club to offset two bogeys.
She ended at 8-under 208, two shots better than three players, and claimed $11,200 for the victory.
Prior to last week, Lee's best finish had been a tie for third place at the Louisiana Pelican Classic.
Ha Na Chae posted a 2-under 70 and tied for second place with Celeste Troche (72) and Beth Allen (71) at 6-under 210. Mollie Fankhauser (71) ended fifth at 5-under 211, and Meredith Duncan (71) was another shot further back in sixth.
Overnight leader Eunjung Yi stumbled to a 76 and dropped into a tie for seventh place at 3-under 213.