 He was the son of Richard Onslow (d. 1719) and Sarah Manning, and grandson of Sir Henry Onslow, of Drungewick, who was a younger son of Sir Richard Onslow.His mother was Sarah, daughter of Thomas Calvert.A member of a junior branch of the Onslow family, which held a powerful Parliamentary interest in Surrey, he married Anne Middleton, the daughter of his second cousin Anne and Thomas Middleton.