William de Percy, 1st Baron Percy, was in the train of William I. After arriving in England following the Harrying of the North (1069–70), he was bestowed modest estates in Yorkshire by Hugh d'Avranches. However, by the reign of Henry II the family was represented by only an heiress, Agnes de Percy (died 1203) following the death of the third feudal baron. As her dowry contained the manor of Topcliffe in Yorkshire, Adeliza of Louvain, the widowed and remarried second wife of Henry I, arranged the marriage of Agnes with her own young half-brother, Joscelin of Louvain. After their wedding, the nobleman from the Duchy of Brabant in the Holy Roman Empire settled in England. He adopted the surname Percy and his descendants were later created Earls of Northumberland. The Percys' line would go on to play a large role in the history of both England and Scotland. As nearly every Percy was a Warden of the Marches, Scottish affairs were often of more concern than those in England.

Adam Rutherford, a twentieth-century geneticist, has claimed that it is "virtually impossible" that a person with a predominantly British ancestry is not descended from Edward III.[2] According to his calculations, "almost every Briton" is "descended between 21 and 24 generations from Edward III".[3]