Very little is known of his career before 1797 or after 1803 except that he was appointed (c.1789) a JP for Armagh and Down, and was defendant in a case at Downpatrick assizes concerning his vote as a potwalloper at Newry (August 1793). Samuel Turner appears to have been sworn into the United Irish movement in January 1797. Such were his talents he became a member of the national directory. Soon he was assisting Bartholomew Teeling (qv) and Alexander Lowry (qv) in their efforts to secure the acquittals of United Irishmen standing trial at the spring assizes in Antrim and Down. Turner enhanced his reputation as a firebrand in an incident at an inn at Newry, Co. Down (16 April 1797), involving the commander-in-chief, the 2nd earl of Carhampton (qv). The latter confronted Turner and reproached him with wearing United Irish colours – a large green necktie; Turner accused him of insolence and called upon him to seize the offending garment, which (according to Edward Newell (qv), who was present) he did; Turner attempted to challenge him to a duel but (according to Newell) Carhampton apologised.
Six weeks later however, after conversing with leading United Irishmen in Dublin, Turner returned to Newry and spoke to the assistant barrister for Co. Down, Joseph Pollock (qv), surrendering himself under a recent proclamation. Outwardly still loyal to the United Irish, Turner was one of the five who represented Ulster at a meeting of United Irishmen in Dublin to consider their state of preparedness for an uprising and reception of a French expedition (c. 5 June). Like other Ulster leaders, threatened with arrest, he fled to the Continent (mid June); he reached the German free city of Hamburg, where, while ostensibly an accredited United Irish agent, he was secretly an agent of the British government. He visited Paris (c.26 July to 19 September), lodged in the rue Fromenteau with Lowry, who had taken refuge in France, and even met the foreign minister, Talleyrand (September). On his return to Hamburg the French ambassador, Jean Frédéric Reinhard, unwittingly gave him a passport enabling him to go to London and meet both Lord Edward FitzGerald (qv), who was at the head of the United Irish national directory, and the 1st marquess of Downshire (qv), a great landowner in Co. Down and a strong upholder of government authority. At his meeting with the marquess (8 October) he offered to tell all he knew of the United Irish not only in Ireland but on the Continent, explaining that though he had joined them from patriotic motives he now saw that ‘the object of the papists was the ruin and destruction of the country and the establishment of a tyranny far worse than what was complained of by the reformers’. Money was also a consideration. He wrote to Downshire asking if ‘Mr Pitt may let me have a cool five-hundred’, much of it for expenses (1 December).
In mid November 1797 Turner returned to Hamburg as a double agent. He was in London again, conferring with Downshire and the duke of Portland (qv), the home secretary (late February to late March 1798); he returned to the Continent, travelling via Hamburg and Holland to Paris, which he reached on 17 April; he again met Talleyrand (20th) but failed to gain an audience with the Directory and left on 9 May, returning to London; he was in Paris on a third visit apparently from late May until late July. Thus he was privy to communications between the United Irish leadership in Dublin and both its agents on the Continent and the French government. Through Downshire he kept the British government informed of United Irish activities on the Continent. He had a hand in the arrests in England of James Coigly (qv) and Arthur O'Connor (qv) (1 March 1798); he exacerbated the division among Irishmen in Paris by supporting James Napper Tandy (qv) against Edward Lewines (qv) (early May); on the arrival in Hamburg of Tandy, James Blackwell (qv), William Corbet (qv) and Hervey Montmorency Morres (qv) he dined with them while secretly playing a part in their arrest (late November).
Turner was named in both the fugitive and the banishment acts (1798), which would have allayed any suspicions of his disloyalty to the United Irish cause; he was also awarded a secret pension of £300 p.a. (1800). Turner was reported by Francis Higgins (qv) to be back in Dublin in March 1801. Whatever the case, he finally left Hamburg late in the summer of 1802; he made for Paris, where (by his own account) he aroused some suspicion among fellow Irishmen and was imprisoned for nine weeks and then detained on parole for eight months; his appearance in Dublin towards the end of 1803 brought his arrest, prompt release and the reversal of the attainder by the court of king's bench (24 November). From then until his death he lived quietly in Dublin, his address appearing in the list of barristers in Wilson's directory as Stephen's Green (1804–10). William John Fitzpatrick (qv) was told by a correspondent in Newry that Samuel Turner was killed about 1810 in a duel on the Isle of Man; but no evidence has been found for this or for any wife or children he may have had.
Suspicions of Turner's perfidy did not become widespread until the publication of the memoirs of James Hope (qv) in the third series of R. R. Madden's (qv) United Irishmen (1846). It remained for details to be supplied by Fitzpatrick (1892) and Paul Weber (1997). Not only was Turner's information ‘ample and accurate’ (Bartlett), he was able to pass it quickly to senior members of the British government; it was especially valuable as Turner was closely connected with the United Irish leadership, knew and even influenced their communications with the Continent and had access to the French foreign minister. In Hamburg and Paris he was in strategic locations in the crucial period of the planning of the United Irish insurrection that occurred in May and June 1798, of which he was therefore able to give warning. The trust placed by the United Irish leadership in one who joined the organisation so late and was so little known is hard to explain, as is its failure even to suspect that he was a double agent. The report he made to the British government at the end of 1797 (National Archives, Kew, HO 100/70, ff 338–52) is an important source for historians on United Irish activity in Ulster and on the Continent in that year.