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 Stakes at Ascot Racecourse. He made much of the running in the race but was outpaced near the finish. Sir Percy, retired with five wins from 10 races and earnings of $2,150,350. He was a Cartier Award nominee at 2 and 3. In the 2005 International Classification for two-year-olds he was assessed on 121 making him the top-ranked colt in Britain and the second highest ranked two-year-old in Europe, three pounds behind George Washington. In the 2006 World Thoroughbred Racehorse Rankings Sir Percy was given a rating of 121, making him the twentieth best racehorse in the world. He was