Theobalds on 14 Dec. 1613 (Metcalfe, Book of Knights, p. 164). Another Sir Thomas Trenchard had in 1509 entertained Philip of Castile when he was driven by a gale in the Channel to take refuge in the port of Weymouth (cf. Grantley, Anecdotes, 1867, i. 329–35). The family traced descent from Paganus Trenchard, who held land in Dorset under Henry I, and from Elizabeth, daughter of Edward I. The Trenchards had intermarried during the seventeenth and preceding century with the Damorels, Moleynses, and Spekes. The politician's father, Thomas Trenchard of Wolverton (1615–1671), married in 1638 Hannah (d. 1691), daughter of Robert Henley of Bramshill, Hampshire. Grace Trenchard, who married Colonel William Sydenham [q. v.], and Jane, who married John Sadler (1615–1674) [q. v.] of Warmwell, both enthusiastic supporters of Oliver Cromwell, were cousins.
John Trenchard matriculated from New College, Oxford, on 15 Aug. 1665. In the same year, according to Wood, he became ‘a probationary fellow of New College in a civilian's place, aged 15 years or more; and entered in the public library as a student in the civil law on 22 Oct. 1668.’ He appears to have taken no degree, but went to the Middle Temple in 1674. He was elected M.P. for Taunton on 20 Feb. 1678–9, and re-elected in the following September (Memb. of Parl. i. 537, 543). His connection with a roundhead and puritan family of such old standing readily procured his admission to the club of revolutionaries which met at the King's Head tavern in Fleet Street (Dangerfield, Narrative of the late Popish Design, 1679, p. 31). Wood says that he was ready to promote ‘Oates his plot, busie against papists, the prerogative, and all that way.’ He became specially intimate with Aaron Smith and the Spekes. In parliament he followed the lead of William Sacheverell and Powle. On 2 Nov. 1680 he spoke against the recognition of the Duke of York as heir-apparent, enouncing the view that ‘to be secured by laws with a popish successor was not practicable.’ He cited the deposition of the queen of Sweden as a precedent, and relied on the navy to check any desire on the part of a foreign potentate to intervene. It was consequently resolved to ‘bring in a bill to disable the Duke of York from inheriting the imperial crown of this realm,’ and in the great debate on 11 Nov. Trenchard contended that the crown was held by statute law, and that, pro bono publico, the parliament must step over any private rights such as those to which James laid claim.
The prominent part which he played on this occasion, and the fact that he had been a regular frequenter of Monmouth's receptions at Soho, acquired Trenchard the reputation of a fierce partisan. He was re-elected for Taunton in March 1681. After the dissolution of the Oxford parliament he put himself, like his friend Aaron Smith, at the disposal of the revolutionary committee, sometime known as ‘The Six.’ He certainly took part in some of the meetings at Sheppards, at which the Rye House plot was concerted in the spring of 1683. He had spoken largely about the hostility to the Stuart dynasty in the west, and especially in Taunton; but when pressed to name a day for a local rising in connection with the plot he pleaded delay. According to Ford, lord Grey of Wark, the pusillanimity which he showed when it was proposed to translate words into action was so great as to provoke merriment among the conspirators (Secret Hist. of the Plot, 1754, pp. 36–7). He was named among the latter by Rumsey and West when they ‘came in’ on 28 June. He was arrested early in July, but owing to the steady refusal of William, lord Russell, to implicate him, and the great skill that he showed under examination, he was ultimately released for want of evidence (cf. Hist. MSS. Comm. 15th Rep. viii. 193). Fearing a rearrest, he spent some time in hiding, and then retired to Dorset. In June 1685, when the news arrived of Monmouth's landing, he was with the Spekes at Ilminster. Instantly recognising his peril, he mounted his horse and advised his friends—among them his brother-in-law, Charles Speke—to do the same. He rode in all haste to Lytchett, but, instead of going to the house, concealed himself in a keeper's lodge. Having obtained the money and papers that he needed, he made his way to Weymouth, and secured a passage thence to the continent. Charles Speke was hanged before his own door. At the urgent request of a common friend Lawton, William Penn, who had already spoken in behalf of Aaron Smith, approached James during the autumn of 1687 with a petition for a free pardon for Trenchard, and a formal pardon was signed by Sunderland in December (ib. 12th Rep. App. vi. 307). Shortly after his return Trenchard was elected M.P. for Dorchester. His parliamentary demeanour was strictly subdued; but early in 1688, as an influential whig who represented accurately the feeling in his county, he was introduced by Penn, along with Treby and some other whigs, to the royal closet. They were urged to speak plainly to the king as to the drift of whig
