William Douglas was the eldest son of James Douglas, 7th Earl of Douglas and Beatrice Sinclair, the daughter of Henry Sinclair, Earl of Orkney.
The power of the Black Douglases, lost after the execution of the 6th Earl, was restored by the 8th Earl, who recovered the lordships of Wigtown, Galloway and Bothwell by marriage (by papal dispensation) to his cousin, Margaret Douglas, Fair Maid of Galloway (daughter of the 5th Earl). He was soon high in favour with James II, and procured the disgrace of Sir William Crichton, presumed murderer of his kinsman the 6th Earl, by an alliance with his rival, Sir Alexander Livingston of Callendar.
The Earl and his party was issued with a Safe-conduct for three years, “to pass through England, to the Marches of Calais and elsewhere in the King of England’s dominions” dated 9 November 1450 Douglas was planning to attend the Jubilee in Rome and would travel via England, Flanders and France. A further Safe-Conduct, this time expressly stating that the Earl could take a party of 100 and naming many of them, was issued (presumably while they were still travelling) on 23 April 1451. The Earl had returned to Scotland by 14 August 1451 as he was the leading Scottish Conservator of the 3-year truce with England, concluded at Newcastle upon Tyne.
During Douglas’s absence in Rome, James II had attacked the lands of the Douglas because of Douglas offences against neighbouring lords. After Douglas’s return, although there was an outward truce, relations continued to be strained between the king’s party and that of the earl.
In early February  Sir William Lauder of Haltoun, a close friend and relative (his mother Helen was a daughter of Archibald, 3rd Earl of Douglas, ‘The Grim’) of Douglas, brought a summons to the Earl to attend the King at Stirling. There was abundant precedent for suspicion in a mandate of this nature, but, as if to allay it, Lauder brought a safe-conduct for Douglas given under the King’s hand in council.
Once there, King James demanded the dissolution of a league into which Douglas had entered with Alexander Lindsay, the ‘Tiger’ Earl of Crawford, and John of Islay. Upon Douglas’s refusal, the king stabbed him as did the several men with the king, and Sir Patrick Gray, according to the Auchinleck Chronicle, “struck out his brains with a pole ax”, and his body was thrown out of a window.
Since Douglas died without issue, his titles passed to his brother James.