Montagu accompanied Prince Charles during his 1623 trip to Habsburg Spain in pursuit of the Spanish Match. He was Member of Parliament for Huntingdonshire in the "Happy Parliament" of 1623–24, the "Useless Parliament" of 1625, and the Parliament of 1625–26. At the time of Charles I's coronation in February 1626, he was made a Knight of the Bath to reward him for his service to Charles in Spain. In May, with help from George Villiers, 1st Duke of Buckingham, Montagu was elevated to the House of Lords, receiving his father's barony of Kimbolton and being styled Viscount Mandeville as a courtesy title, since his father had been created Earl of Manchester in February when Parliament convened. His first wife, who was related to the Duke of Buckingham, having died in 1625 after two years of marriage, Mandeville married in 1626 Anne Rich, daughter of Robert Rich, 2nd Earl of Warwick. The influence of his father-in-law, who was afterwards admiral on the side of the parliament, drew Mandeville to the popular side in the questions in dispute with the crown, and at the beginning of the Long Parliament he was one of the recognised leaders of the popular party in the Upper House, his name being joined with those of the Five Members of the House of Commons impeached by the king in 1642. At the outbreak of the Civil War, having succeeded his father in the earldom in November 1642, Manchester commanded a regiment in the army of Robert Devereux, 3rd Earl of Essex, and in August 1643 he was appointed Major-General of the parliamentary forces in the eastern counties, with Cromwell as his second in command. He soon appointed his provost-marshal, William Dowsing, as a paid iconoclast, touring the churches of Suffolk and Cambridgeshire destroying all "Popish" and "superstitious" imagery, as well as features such as altar-rails. Having become a member of the Committee of Both Kingdoms in 1644, he was in supreme command at the Battle of Marston Moor but in the subsequent operations his lack of energy brought him into disagreement with Cromwell, and in November 1644 he strongly expressed his disapproval of continuing the war. Cromwell brought the shortcomings of Manchester before Parliament in the autumn of 1644 and in April the following year, anticipating the Self-denying Ordinance, Manchester resigned his command. He took a leading part in the frequent negotiations for an arrangement with Charles, was custodian with William Lenthall of the Great Seal from 1646 to 1648, and frequently presided in the House of Lords. He opposed the trial of the king, and retired from public life during the Commonwealth but after the Restoration, which he actively assisted, he was loaded with honours by Charles II. In 1660, he was nominated by the House of Lords to be one of the Commissioners for the Great Seal of England. In 1667 he was made a General, and he died on 5 May 1671. Manchester was made a Knight of the Order of the Garter in 1661, and became a Fellow of the Royal Society in 1667. Men of such divergent sympathies as Baxter, Burnet and Clarendon agreed in describing Manchester as a lovable and virtuous man, who loved peace and moderation both in politics and religion.
Personal life
Lord Manchester was five times married, leaving children by two of his wives. He married firstly, Susannah Hill on 6 February 1623. She was a daughter of John Hill and Dorothy Hill. Through her aunt, Susannah was a first cousin of George Villiers, 1st Duke of Buckingham. They had no children. He married secondly, 1 July 1625, Lady Anne Rich, daughter of Robert Rich, 2nd Earl of Warwick and the former Frances Hatton. Before her death on 16 February 1642, they were the parents of three children:
On 20 December 1642, ten months after the death of his second wife, he married her first cousin, Essex, Lady Bevill. She was the widow of Sir Thomas Bevil and a daughter of Sir Thomas Cheek and the former Lady Essex Rich. Before her death on 28 September 1658, they had a daughter:
Manchester was portrayed by actor Robert Morley in the 1970 film Cromwell. He is inaccurately depicted sitting in the House of Commons in Cromwell's presence although he had been a member of the Lords since 1626.