The Sir Mark Prescott-trained four-year-old engaged in a thrilling tussle with red-hot favourite Lady Aurelia on the Knavesmire and though Frankie Dettori punched the air after crossing the line aboard the American filly, a photo-finish confirmed Marsha had claimed victory by a nose.
As a dual Group One winner, Marsha will have plenty of suitors when she heads to the sales later in the year, but she will first return to France on Arc weekend.
Dan Downie, racing manager for owners the Elite Racing Club, said: "She's good and fine. She'll go to France again in early October.
"The plan is for her to still go through the ring at the end of the year.
"She's been a wonderful filly for us and she has achieved so much."