World number two Lewis went into the final round seven strokes behind Lee, needing a miracle to overturn the defect.
She had closed the gap to two shots after playing the first 15 holes in five-under-par, but no one could have foreseen what was to follow.
She sunk a 25-foot putt on the 16th green for birdie, and followed it with another from 12 feet on the next hole to tie for the lead.
Her remarkable run continued on the final hole. She hit her approach to 25 feet and made another birdie putt to card a closing 64, good enough to secure her fourth LPGA Tour victory of the year.
“It was pretty unexpected to make those last three birdies. They were all pretty long putts so it wasn’t easy,” Lewis said.
“I knew I was a ways back, but I thought if I got in to 10-under, it would be close. To shoot a 64 on the final day is always good.”
Her victory marks the first time in 25 years that a Mizuno staff player has won a Mizuno-sponsored event.
Lewis will now contest the Lorena Ochoa Invitational and the Title Holders, knowing a victory in either will almost certainly earn her the Player of the Year accolade.