William Douglas lived at Cluny, Fife, Scotland.
William Douglas was appointed guardian to King James III. during his minority, and received from him, before 1462, some of the spoil accruing from the forfeiture of the Earl of Douglas and his adherents. Two years later, in 1464, King James appointed him Warden Douglas of the Eastern and Middle Marches, in succession to his brother the 4th Earl of Angus before 1475, and at the same time committed to him the keeping of the castles of Douglas and Tantallon, with their lordships. When the 6th earl, " Bell-the-Cat ", came of age in 1470, William Douglas came before the King and ad eius genua prouolutus — resigned ward of Tantallon and the lordship of Douglas per fustem et bacnlum in the said earl's favour. The lands which he received upon the forfeiture of his kinsmen, comprising Sunderlandhall in Selkirkshire, Cranston in Midlothian, and Traquair and Leithenhope in Peeblesshire, were erected in 1464 into the barony of Sunderland in his favour.
William Douglas of Cluny, sometimes styled lord of Sunderland and sometimes lord of Traquair, died, probably unmarried, before 1475, when his lands of Cluny appear in possession of the 5th Earl of Angus.
The estate of Traquair was originally a royal domain, and was conferred by Robert Bruce on his warm friend and devoted adherent, Lord James Douglas. After passing through various hands, it came into possession of an ancestor of the Murrays of Elibank, and was forfeited by William Murray in 1464. It was given to William Douglas of Cluny, but was almost immediately thereafter assigned to the Boyds.