In addition to his Earldom of Douglas he was Earl of Wigtown, Lord of Galloway, Lord of Bothwell, Selkirk and Ettrick Forest, Eskdale, Lauderdale, and Annandale in Scotland, and de jure Duke of Touraine, Count of Longueville, and Lord of Dun-le-roi in France. He was the eldest son of Archibald Douglas, 5th Earl of Douglas and Lady Eupheme Graham.
He married Lady Janet Lindsay, daughter of David, Earl of Crawford, and succeeded to the earldom on the death of his father, who had served as regent of James II. Following Archibald Douglas’s death, Sir William Crichton, Sir Alexander Livingston of Callendar, and James Douglas, Earl of Avondale (William Douglas’s great-uncle) shared power. Together they conspired to break the power of the late Archibald Douglas’s family, and summoned William and his younger brother David to Edinburgh Castle. The so-called ‘Black Dinner’ which followed saw the two boys summarily beheaded on trumped-up charges, over the protests of the young King James II.
The lordships of Annandale and Bothwell fell to the crown; Galloway to Margaret Douglas (William Douglas’s sister), and the Douglas lands and earldom passed to William’s great-uncle James Douglas, the Earl of Avondale, who was accordingly seen later as the main perpetrator.
The title passed to James Douglas, 7th Earl before passing to his son, William. The Black Douglas’ power was largely lost between the reign of the 6th and 8th Earls.