William was the son of Robert Arbuthnot of Haddo by his spouse Mary née Urquhart; elder brother of George Arbuthnot, 1st of Elderslie.
William was the son of Robert Keith Arbuthnot FRSE (1728-1803) of Haddo Rattray, and Mary Urquhart of Cromarty. He was the elder brother of George Arbuthnot, 1st of Elderslie.