1780 he was elected moderator of the general synod of Ulster at Dungannon, co. Tyrone. Though the contrary has been stated, Dickson was not a member of the volunteer conventions at Dungannon in 1782 and 1783. He threw himself heart and soul into the famous election for county Down in August 1783, when the houses of Hill and Stewart, representing the court and country parties, first came into collision. Dickson, with his forty mounted freeholders, failed to secure the re-election of Robert Stewart, who eventually took refuge ‘under the shade of a peerage.’ But in 1790 he successfully exerted himself for the return of Stewart's son (also Robert), better known as Lord Castlereagh. Castlereagh proved his gratitude by referring at a later date to Dickson's popularity in 1790, as proof that he was ‘a very dangerous person to leave at liberty.’ In 1788 Dickson was a candidate for the agency of the regium donum, but the post was conferred on Robert Black [q. v.]
As early as December 1791, Dickson, who was now a D.D. of Glasgow, took the test as a member of the first society of United Irishmen, organised in October at Belfast by Theobald Wolfe Tone. He labours to prove that he attended no further meetings of this body, devoting himself to spreading its principles among the volunteer associations, in opposition to the ‘demi-patriotic’ views of the whig clubs. At a great volunteer meeting in Belfast on 14 July 1792 he opposed a resolution for the gradual removal of catholic disabilities, and assisted in obtaining a unanimous pledge in favour of total and immediate emancipation. Parish and county meetings were held throughout Ulster, culminating in a provincial convention at Dungannon on 15 Feb. 1793. Dickson had been a leading spirit at many of the preliminary meetings, and, as a delegate from the barony of Ards, he had a chief hand in the preparation of the Dungannon resolutions. Their avowed object was to strengthen the throne and give vitality to the constitution by ‘a complete and radical reform.’ Dickson was nominated on a committee of thirty to summon a national convention. Before he left Dungannon he was called upon for a sermon to the times, and had an immense audience, the established and catholic clergy being present. The Irish parliament went no further in the direction of emancipation than the Relief Act (33 Geo. III, c. 21), which received the royal assent on 9 April, and remained unextended till 1829; while the passing of Lord Clare's Convention Act (33 Geo. III, c. 29), still in force, made illegal all future assemblies of delegates ‘purporting to represent the people, or any description of the people.’
The Convention Act put an end to the existence of the volunteers as a political party; those who were disinclined to accept the situation became more and more identified with the illegal operations of the United Irishmen. Dickson got up political meetings and preached political sermons, which were considered ‘fraught with phlogistick principles’ (Musgrave). He maintains that he exerted himself to prevent outbreak, and that ‘reform alone was sought for.’ In October 1796 several members of his congregation were arrested, and a reward of 1,000l. was offered to one Carr, a weaver, for evidence which would secure Dickson's conviction The suspects were liberated without trial at the summer assize in Downpatrick, 1797; and Dickson, though a watch was kept on his movements, would have been safe but for his own folly. In March and April 1798 he was in Scotland arranging family affairs. During his absence the plan of the northern insurrection was digested, and Dickson soon after his return agreed to take the place of Thomas Russell as ‘adjutant-general of the United Irish forces for county Down.’ This appointment he does not deny, though with great ingenuity he disposes of the insufficient evidence brought forward in proof of it: ‘I may have been a general for aught that appears to the contrary; and I may not have been a general, though people said I was.’ A few days before the projected insurrection he was arrested at Ballynahinch. The date of the arrest has been variously stated, but his own very circumstantial narrative fixes it on Tuesday evening, 5 June. He was conveyed to Belfast, and lodged in the ‘black hole’ and other prisons, till on 12 Aug. he was removed to the prison ship, and detained there amid considerable discomfort till 25 March 1799. From Ireland he was transferred to Fort George, Inverness-shire, arriving there on 9 April. Here, with his fellow-prisoners, he was exceedingly well treated. His liberty was offered him on condition of emigration, but he demanded a trial, which was never granted. At length, on 30 Dec. 1801, he was brought back from Fort George, and given his freedom in Belfast on 13 Jan. 1802.
Dickson returned to liberty and misfortune. His wife had long been a helpless invalid, his eldest son was dead, his prospects were ruined. With fierce humour he reckons his losses at 3,618l., and sets down his compensation as 0,000l. His congregation at Portaferry had been declared vacant on 28 Nov. 1799. William Moreland, who
