Margot Did won the Nunthorpe Stakes at York's Ebor Festival to give jockey Hayley Turner the second Group One success of her career.
The 20-1 shot, trained by Michael Bell, came home three-quarters of a length ahead of Hamish McGonagall (28-1), with Prohibit (12-1) third.
who won Newmarket's July Cup on Dream Ahead,
said: "I can't believe it - it's the best season ever.
"It just goes to show if you work hard and are dedicated, it can be done."
The 28-year-old, the only female jockey to have won a Group One race outright, praised her boss Bell by adding: "It must have been difficult for him when I first started, pushing the owners to put a girl on their horses.
DID YOU KNOW?
Alex Greaves is the only other woman to win a Group One race - she dead-heated for first on Ya Malak in the Nunthorpe Stakes in 1997
"Hopefully I can repay him by riding winners like this."
Hamish McGonagall and Masamah showed plenty of early speed but, with the runners spread right across the track, Margot Did blitzed through.
Heavily backed favourite Hoof It (11-4) never threatened to figure in the finish.
Bell said: "This filly won a Listed race at Sandown by five lengths. Not many sprints are won by five lengths. She looked good that day and this was the plan.
"She's a really tough filly and we've found the key to her.
"Today from a long way out she was one of the few on the bridle. It's great."