William Heveningham was the eldest son of Sir John Heveningham of Ketteringham, Norfolk. After attending Pembroke College, Cambridge, he married Catherine (d. 1648), daughter of Sir Henry Wallop MP, an extremely wealthy and influential resident of Hampshire and Wiltshire. His second marriage (1650) was to Mary Carey, daughter of the Earl of Dover. Heveningham inherited substantial properties on his father's death in 1633. He was elected MP for Stockbridge in Hampshire, in the Short Parliament and again in the Long Parliament (1640)
Heveningham was active in the administration of East Anglia for Parliament during the civil wars and advanced money for the support of the parliamentarian garrison at Newport Pagnell in Buckinghamshire. Although he was an elder of the Presbyterian classis of Dunwich, Heveningham was associated with the Independent faction in Parliament and in January 1649 was appointed a commissioner on the High Court of Justice for the trial of King Charles. Although he attended every session of the trial, he refused to sign the King's death warrant.
During the Commonwealth and Protectorate, Heveningham bought up many properties confiscated from the Church and from Royalists, and also speculated in buying army debentures. He was a member of the Council of State in 1649 and 1650.
Heveningham was probably the first of the regicides to surrender to the authorities at the Restoration in 1660. He was brought to trial in October 1660, found guilty of treason for his part in the King's trial and sentenced to death. However, he successfully petitioned for mercy, claiming that he had tried to prevent the King's execution, had opposed Cromwell's tyranny and had contributed £500 to Booth's Uprising in 1659. He was imprisoned at Windsor Castle, where he remained until his death in 1678.
References:
Daniel Webster Hollis, William Heveningham, Oxford DNB, 2004