Sir Percy dam Percy's Lass won the Group 3 September Stakes in 1988 (and subsequently died of colic when Sir Percy was a foal. Sir Percy was the last of her offspring) and his damsire Blakeney won the Epsom Derby himself in 1969. Sir Percy was named after Sir Percy Blakeney, the hero of the novel "The Scarlet Pimpernel". In November 2003 Sir Percy was sent as a weanling to the Tattersalls sales. He was bought for 20,000 guineas by Will Edmeades bloodstock, acting on behalf of Anthony Pakenham. Pakenham bought the colt as compensation for losing a National Hunt Horse. Sir
Sir Percy Dwyer called Sir Percy an exceptional horse with a "tremendous turn of foot". Martin Dwyer's ride on Sir Percy at Epsom earned him the 2006 Lester Award for Flat Ride of the Year. Sir Percy was troubled with a series of training problems which included a shoulder injury which ruled him out of a run in the Prix de l'Arc de Triomphe. On his only start after his Derby win he finished seventh of eight runners behind Pride in the Champion Stakes, when it was revealed he was still suffering the ill effects of an injury sustained in the Derby.