After an early marriage William Sloper formed a connexion in 1737 with Susannah Maria Cibber, the well-known actress, apparently with the connivance of her husband, Theophilus Cibber, the actor. Cibber, however, brought two actions against Sloper in 1738 and 1739 for crim. con. and detention of his wife, as a result of which he obtained damages of £20 and £500. By Susannah, Sloper had a son and a daughter, who both died young.
In 1747 Sloper stood as a Whig for his father’s old borough, for which he was seated by the House of Commons after a double return, largely owing to the efforts on his behalf of Henry Fox.