Tremayne was returned unopposed for the county at the early age of 26: he did not have a considerable estate, but his clerical father had the expectation of one, and to quote Davies Gilbert writing his county history in 1838,
so high and so extensive was the reputation of Mr Tremayne [senior] throughout the whole county that his son, buoyant on the father’s virtues, and before opportunities were afforded for displaying his own, passed by a unanimous election into the high station of representative for Cornwall, but experience soon proved that Mr John Hearle Tremayne wanted no assistance from hereditary claims to make him worthy of that, or any other distinction.
Hist. Cornw. i. 423. Tremayne, on inheriting, was worth £12,000 p.a. (West Briton, 27 Apr. 1882).
Tremayne, who was returned with Sir William Lemon, his future father-in-law, gave no account of his politics at his nomination in 1806 and none was asked for; he kept Lord Grenville guessing as to his politics and he voted against his administration at least once, on the Hampshire election, 13 Feb. 1807. Yet he requested and received the assurance of Grenville’s support in the 1807 election,
On 21 June 1813 Tremayne intervened in the debate on the Helston election, exonerating the Cornish clergy from the blame attached to them by Henry Swann and proposing that, rather than enfranchise the ‘lowest classes’ in the borough, the House should enfranchise the hundred. It was not until 1815 that he began to appear occasionally in the minority lobby again, supporting Romilly’s motion against the employment of the militia in peacetime, 28 Feb., and Tierney’s motion on the civil list, 8 May. He also opposed the grants to the royal family, May-July 1815. In March, April and May 1816 he voted and spoke in favour of reducing public expenditure and continued to do so for the remainder of the period in the interests of retrenchment. On 18 Mar. 1816, when he voted for the property tax, he had presented a county petition for a modified tax on property in preference to the continuation of the wartime malt tax, and on 24 Apr. he backed his father-in-law’s petition from Cornwall for retrenchment and economy, taking exception however to its allegation of a ‘breach of faith’ by government. On 7 Feb. 1817 he was appointed to the finance committee: he had been twice defeated on a vote, but the withdrawal of several Whigs left vacancies. On 25 Apr. he favoured a committee on the salt duties, defending the interests of the West of England fisheries, and in the ensuing Parliament he twice spoke in favour of reducing the duties. Although he voted against the appointment of a secret committee, 5 June, he was in the majority in favour of habeas corpus, 23 June 1817.
Tremayne was involved, briefly, in the formation of the Grenville ‘third party’ in January 1818, when Joseph Phillimore, their adjutant in this affair, reported:
Tremayne after questioning me whether we took our seats where we did, accidentally or designedly, on my explaining the state of things to him, expressed great satisfaction. He is a thoroughly independent person, and wholly unconnected with any party, but I should not be at all surprised if he were to think and act as we do, on most occasions, without decidedly attaching himself to our squad.
Charles Williams Wynn added, 2 Feb. 1818, ‘Tremayne has told Phillimore that he only wondered I had not changed my bench sooner, and has both days, that he has been down, sat with us’. (He had supported Williams Wynn’s unsuccessful candidature for the Speakership, 2 June 1817.)
Tremayne refused to pledge himself to reform of Parliament to meet the request of Cornish reformers.
