Maynard did not enter the Long parliament until 1647, when he was returned for Lostwithiel, 20 Jan. At heart a royalist, he became conspicuous as a leader of the presbyterian party in the struggle with the army, and was accordingly included in the eleven members charged with disaffection by Fairfax on 16 June 1647. After the outbreak of mob violence, by which, on 26 July, the recent militia ordinance was rescinded, and the command of the London trained bands restored to the lord mayor and corporation, he was readmitted to the House of Commons and placed on the committee of safety [cf. Gayee, Sir John, d. 1649, and Glynne, Sir John, 1603-1666]. When the army gained the ascendency," he was charged with unlawfully levying an armed force within the city, was arrested by a general warrant under the speaker's hand, and committed to the Tower (8 Sept.) during pleasure of parliament. An impeachment of high treason followed on 1 Feb. 1647-8. Maynard replied by a letter to the speaker, 4 Feb., in which he refused to make any defence, and claimed to be tried by a jury, citing Magna Charta and the Petition of Right. This claim he reiterated at the bar of the House of Lords on the following day, refusing to kneel or in any way recognise the jurisdiction of the house. He was accordingly fined 500l. and remanded to the Tower. Thence he issued several protests against the claim of the House of Lords to jurisdiction over commoners, and on 19 Feb., being again brought to the bar of the House of Lords, he repeated his former tactics, and was again remanded. He remained in the Tower until 3 June, when he was set at liberty, and resumed his seat in the commons. On 27 June he spoke in support of the 'city petition' for a 'personal treaty Avith his Majesty.' On 27 July he pleaded the cause of John Lilburne [q.v.] in an able speech which procured his release.
Maynard had estates at Walthamstow, Tooting, Bradford, Yorkshire, and Isleham, Cambridgeshire. His town house was 'The Portcullis,' Russell Street, Covent Garden. In 1649 he argued at length in the exchequer chamber before the committee on the scheme for draining the Bedford level, which he opposed as encroaching upon proprietary rights. He also laid (3 July 1653) a petition against the scheme before the commissioners charged with supervising its execution. The petition, with a schedule of exceptions to the act of parliament (1649, c. 29) authorising the work, was published, and elicited 'An Answer to a printed Paper dispersed by Sir John Maynard, entituled the Humble Petition of the Owners and Commoners of the towne of Isleham,' &c., London, 1653. 4to.
Maynard died on 29 July 1658, and was buried in the churchyard of Tooting Graveney. By his wife, Mary, daughter of Sir Thomas Middleton of Stansted Mountfitchet, lord mayor of London (who survived him), he had issue a son, John, who was knighted 7 June 1660, and died 14 May 1664.
There are extant by Maynard : 1. 'The Copy of a Letter addressed to the Father Rector at Brussels, found among some Jesuites taken at London about the third yeere of his Majesty's Raigne. Wherein is manifested that the Jesuites from time to time have been the only Incendiaries and Contrivers of the Miseries and Distractions of this Kingdom,' London, 1643, 4to. Other versions are in Prynne's 'Hidden Works of Darkness,' London, 1646, fol.; Rushworth's 'Historical Collections,' i. 474-6 ; and 'Camden Society's Miscellany,' ii. and iv. Supplement, note ad Jin. 2. 'The 'Humble Plea and Protest of Sir John Maynard (a late Member of the hon. House of Commons) to the Speaker of the House of Peeres,' London, February 1647-8, fol. 3. 'England's Champion : or the Just Man's Fortitude manifested in that gallant resolution of Sir John Maynard (late Member of the House of Commons). Being the Copie of his Letter and Protest sent into the Lords, 14 Feb. 1647,' London, 1648 fol. 4. 'A Speech spoken by an hon. Knight in the House of Commons, upon the delivery of the City Petition, being Tuesday, the 27th of June 1648,' London, 1648, 4to. 5. 'A Speech spoken in the hon. House of Commons, by Sir John Maynard, &c. Wherein he hath stated the Case of Lieut.-col. John Lilburne,' &c., London, 1648, 4to ; reprinted in 'Parliamentary History,' iii. 959 et seq. 6. 'The Picklock of the old Fenne Project,' London, 1650, 4to (the substance of Maynard's argument in the exchequer chamber against the draining of the Bedford level).
[Baker's Northamptonshire, ii. 190; Visitations of Essex (Harl.Soc.).ii. 679; Wood's Athenæ Oxon. iv. 296 ; Morant's Essex, ii. 432; Collins's Peerage (Brydges), vi. 284 ; Manning and Bray's Surrey, iii. 489 ; Inner Temple Books ; Commons' Debates in 1625 (Camden Soc.), p. 108 ; Returns of Members of Parliament (Official); Nicolas's Hist. Brit. Knighthood, vol. iii., Chron. List, xvi ; Metcalfe's Book of Knights, p. 186 ; Burke's Extinct Baronetage ; Le Neve's Pedigrees of Knights (Harl. Sec.), p. 73; Cal. State Papers, Dom. 1623-5 pp. 115, 330, 1628-9 p. 186, 1637 p. 237, 1641-3 p. 505, 1644-5, 1653-1654; Cal. Comm. Advance of Money, Dom. 1642-56, pt, ii. p. 834 ; Cobbett's State Trials,
