Both shot closing rounds of 71 to finish at nine under par on 283, one stroke clear of Britain's Joanne Morley, the holder. Scotland's Dale Reid was a further stroke behind.
It was a first win for the 27-year-old Mills, who thought that her chances of victory had vanished when she hit her three iron second shot into the lake at the 441-yard 18th hole. She dropped clear then pitched to five feet to salvage a par. Brooky, playing in the game behind, was on the green in two but then took three putts to finish in a tie with Mills.
They went back to the same hole for the play-off, where they both made birdie fours. Playing the same hole again, Mills almost found the water again but from 70 yards she pitched into the hole for a winning eagle three.
"I'm just speechless. I can't believe I have won. It will take some time to sink in, but I will never forget the lake," said Mills after collecting her winner's cheque.
Morley put up a great defence of her title but failed to make a birdie over the last nine holes in her round of 74. Her best chance was at the final hole, where she pitched to five feet but narrowly missed the putt she needed to force her way into the play-off.
Brandie Burton, who has not won a tournament since 1993, shot a six- under-par 66 to take a two-stroke lead over Laura Davies after the second round of the Giant Eagle LPGA Classic. Burton had a 13-under-par total of 131 on the Avalon Lakes Golf Course. Davies also shot a 66, while the first-round co-leader Tammie Green (71) was four back, along with Moira Dunn (68).Reuse content