Feng, from Guangzhou, China, took a one-stroke lead into the final round but Lewis led by a stroke after a birdie at the 13th hole. Both players had bogeys at the 14th and birdies at No. 16.
Feng went for the green on the par-5 closing hole, hitting the flagstick with her second shot. She was left with a short putt for eagle.
That gave her a final round score of 5-under-par 68, moving her to 26-under 266 for the tournament. Lewis was second at 25-under. Inbee Park who shot 66 Sunday, finished third at 21-under and Karrie Webb, after a final round 67, was fourth at 20-under.
Lewis had four birdies in the first six holes Sunday in taking the lead. She finished with six birdies and the bogey at 14 on the day. Feng, coming off back-to-back bogey-free 64s, had four birdies and one bogey before the eagle in her 68.
It was the second LPGA title for Feng, who won the LPGA Championship in 2012. She has 10 wins as a professional overall. The Reignwood title comes with a winner's check for $270,000.