[4] On 20 November 1688, he was appointed colonel of the 1st Regiment of Horse in place of his brother, James, but was removed the following month.

On 4 August 1646, before his parents' marriage, his father was created the Lord Daer and Shortcleuch and Earl of Selkirk, with remainder to his heirs male whatsoever bearing the name of Douglas.

His father then resigned the Lordship of Daer and Earldom of Selkirk into the King's hands, who conferred them by novodamus dated 6 October 1688 with the precedency of 1646 upon William's younger son, Charles and his four younger brothers, John, George, Basil and Archibald, respectively in tail male.

[4] Lord Selkirk died, unmarried,[7][b] in London on 13 March 1739 and was succeeded in the earldom by his younger brother, John.

He was buried on 18 April 1739 in the vault of the Duke of Buckingham in Henry VII Chapel at Westminster Abbey.