Sir William Clarke (died 1666) was an English politician. Born about in London of obscure parentage, he was admitted as a student to Inner Temple in and called to the bar in On 28 Jan he was appointed Secretary at War, after having served for at least the previous twelve years as secretary to General Monck. Clarke served as Secretary to the Council of the Army, 1647–1649, and Secretary to General Monck and the Commanders of the Army in Scotland, Clarke served as the Secretary at War from to his death in 1666, as a casualty of war with the Dutch. With the Restoration, both Monck and Clarke had great favour with Charles II, who bestowed knighthood upon Clarke and gave him the use, for a term of some years, of the great lodge and 60 acres within Marylebone Park. His widow was Dorothy Clarke, and they had one son, George Clarke.