Trenchard’s marriage settlement provided him with a seat at Warmwell, which he rebuilt on an unconventional Y-plan, and the Dorset manor of Bestwall, just outside Wareham.35 Hutchins, i. 415-16; J. Newman and N. Pevsner, Dorset (Buildings of Eng.), 440. He was first returned for the borough in 1620, but left no trace on the records of the third Jacobean Parliament. Re-elected in 1624, when the prominent Dorset lawyer Sir Francis Ashley* canvassed on his behalf,36 Add. 29974, f. 76. he was personally named to legislative committees concerned with poor relief and the estates of Sir Reginald Mohun* (16 Mar. and 8 May). He also attended one meeting of the committee for the bill to confirm the customs of Beaminster Secunda, Dorset.37 CJ, i. 687a, 701a; C.R. Kyle, ‘Attendance Lists’, PPE 1604-48 ed. Kyle, 201. Although again returned for Wareham to the 1625 Parliament, he played no known part in its proceedings, and is not known to have stood in the next two parliamentary elections. However, in 1628 he may have influenced the return at Wareham of his cousin, Sir John Meller.38 Hutchins, iii. 326; Vis. Hants (Harl. Soc. lxiv), 10.
Trenchard compounded for knighthood in Dorset in 1631, but as a servant of the 4th earl of Bedford he was chiefly employed during this decade in the development of the Covent Garden estate, where he was resident by 1635.39 Som. and Dorset N and Q, iv. 18; St. Paul, Covent Garden, 33; M.F. Keeler, Long Parl. 364. Twice more elected at Wareham in 1640, he was active both in the Long Parliament and in the parliamentarian cause at local level. Though a commissioner for compounding, he refused to act as one of the king’s judges in 1649.40 Keeler, 364. Trenchard sat in three more Parliaments during the Interregnum. He died in London in October 1662, and was buried at Warmwell. No will or grant of administration has been found. The Warmwell estate passed to his son-in-law John Sadler†.41 E178/6204; Soc. Gen., Warmwell par. reg.