His sire, Prince Rose, stood at the Haras de Cheffreville stud farm in France and was mated to the mare Cosquilla. When World War II broke out, the pregnant mare was shipped to Ireland, where she gave birth to Princequillo. Considering the danger from Germanbombing and the likelihood there would be no racing for some considerable time, Cosquilla's owners shipped her and her colt to the United States.
In July 1942, Princequillo made his American racing debut. After a few races, he was purchased by Boone Hall Stable, owned by Prince Dimitri Djordjadze of Georgia and his American-born wife, Audrey Emery. They placed him under the care of future Hall of Fame trainerHoratio Luro. Princequillo won several important races at longer distances. He broke the Saratoga Race Course record for 1¾ miles and his performances were such that he is considered to be the best long-distance runner, with the exception of Kelso, in American racing history.