Trained by her eccentric and unconventional Italian owner, she won the Derby as a 100/1 outsider, creating one of the biggest upsets in racing history.

She was bred, owned and trained by Cavaliere Edoardo Ginistrelli (1833–1920), who had come to England from Italy in the early 1880s after selling his stud and stable near Mount Vesuvius.

At the same stables in Newmarket, Suffolk was a horse called Chaleureux, a nine guinea stallion, which was used as a 'teaser', i.e. his job was to detect when mares came into season.

A week before the Derby Lord Alfred Douglas dreamt that Signorinetta would win and so a placed a five-pound bet on her, despite friends telling him not to.

[3] On 3 June 1908, ridden by Billy Bullock, she started at odds of 100/1 against seventeen colts for The Derby in front of a huge crowd which included the King and Queen and several other members of the Royal Family.

Signorinetta was not among the early leaders but she began to make rapid progress from half way and hit the front two furlongs from the finish.

Despite the mile and a half trip and the exceptionally hot weather, the filly looked as though she had hardly exerted herself afterwards: according to one report "she would not have blown a pinch of snuff off a sixpence".

Half a mile into the race Signorinetta's task was made considerably easier when French Partridge fell and brought down Rhodora.

Signorinetta raced in second place until the turn into the straight, when she moved into the lead and ran on strongly to win easily by three quarters of a length from Lord Falmouth's filly Courtesy, with Santeve third.