Karen Stupples, who held a three-shot lead heading into the final round, had a 10-foot putt at the last to force a playoff but was unable to convert. Stupples, who won the Women's British Open last week, managed a closing 73 to join four-time Jamie Farr winner Se Ri Pak at 6-under-par 278.
Mallon added another win to her 2004 season that has already seen her title at the U.S. Women's Open and the Canadian Women's Open. The 41-year-old is having her best season since 1991, when she titled four times on the LPGA Tour.
'I just tried to hang in there today,' said Mallon. 'You never know what's going to happen on this golf course.'
She opened her round at Highland Meadows Golf Club with a birdie at the par-3 second after her tee shot landed within 7 feet of the hole. Mallon then birdied the sixth to get even closer to Stupples, but back-to-back bogeys from the 11th meant she had to attempt the climb up the leaderboard once more.
Mallon persisted and chipped in for a birdie at the par-4 15th. She then grabbed a share of the lead with a birdie at the par-4 16th and dropped her third shot inside eight feet at the par-5 18th.
Mallon ran home the birdie putt at the last to secure her 18th career victory on the LPGA Tour.
Stupples, who was also seeking her third win of the season, parred her first five holes of the day before finding trouble with a bogey at the sixth. She countered with a birdie at the very next hole, but stumbled again on the back side with back-to-back bogeys starting at the 12th.
With Mallon and Pak charging, Stupples was unable to regain the outright lead down the stretch. She gave herself a chance on 18, but her birdie try missed left.
Pak made a late run with three birdies and a bogey on the front side. The Korean faltered with a bogey at the 10th but she responded with a birdie at the 11th to get back to minus-5.
Pak spun her approach back to 9 feet for a birdie at the 16th, but was unable to birdie either of the closing par-5s to join Stupples in second after a round of 68.
Rachel Teske, who captured this event in 2002, posted a 68 to take fourth place at 5-under-par 279. Lorie Kane was one shot further back at 4-under-par 280.