Mallon wound up at seven-under-par 277 for her third victory of the season.
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 missed. Stupples, who won the Women's British Open last week, had a closing 73 to join four-time Jamie Farr winner Se Ri Pak at six-under-par 278.
Mallon had already won this year's U.S. Women's Open and the Canadian Women's Open. The 41-year-old is having her best season since 1991, when she won four times on the LPGA Tour.
"I just tried to hang in there today," said Mallon. "You never know what's going to happen."
Mallon won her 18th career victory on the LPGA Tour.