McMahon (McMechan), Arthur (1755?–1816?), presbyterian minister, United Irishman, and officer in the French army, was born, probably in 1755, at Donanelly in the parish of Inch, Co. Down; he was the younger son of Alexander McMechan (d. c.1793), a prosperous farmer there. He matriculated at Glasgow University as McMechan (1773) and was known by this name until the mid 1790s. Having graduated MA (1777), he was licensed by the presbytery of Antrim but later joined the presbytery of Killyleagh (a heterodox presbyterian synod); while still a licentiate, he was called to the presbyterian congregation at Kilrea, Co. Londonderry, and was ordained minister on 12 October 1789. He settled with his family on a farm of 60 Irish acres nearby at Lisnagrot on the estate of Alexander Stewart. According to a contemporary who knew him, Charles Kennedy, he was a ‘supposed tutor in the Londonderry family and of very inferior gifts as a preacher’, who was ‘compulsorily forced upon the congregation by the influence of Alexander Stewart, the landlord, and his agents’, eventually ‘establishing a character there as a most daring and pugnacious man, impatient of all defiance and opposition’ (Rodgers and Bigger). By McMahon's own account the Stewarts turned against him and caused him difficulty on his farm after his father voted against Robert Stewart, the future Viscount Castlereagh (qv), at the Co. Down election of 1790. He moved in October 1794 to a congregation at Holywood, Co. Down, and rejoined the presbytery of Antrim.
McMahon became involved in the United Irish movement – by 1796 radicalised and militarised – and rose to be one of the seven colonels in the county. In mid-June 1797 he attended a meeting of United Irishmen at Randalstown, Co. Antrim, at which, French assistance not having arrived, he urged an immediate insurrection. Two weeks later he fled to Scotland, crossed into England, met other Irish conspirators and English radicals in London, and on 8 July left Dover with James Coigly (qv) for Hamburg. He was mentioned by Theobald Wolfe Tone (qv) on 15 October 1797 as residing in Paris. Tone and James Napper Tandy (qv) were in rival camps of the United Irishmen; McMahon joined Tandy's and became an aide to the old radical. After news of rebellion in Ireland reached France (June 1798), McMahon and another Ulster United Irishman, Joseph Orr, prepared to go to Ireland as emissaries to assure Ulster that a French force would arrive to assist. Instead, Orr joined Tandy on his expedition to the coast of Donegal and McMahon, by one account, embarked at Flushing on a small corvette, which sprang a leak and, pursued by British cruisers, barely made it back.
For the next three or four years he lived obscurely and poorly in Paris. Assertions that he was an informer to the British government, accepted by Rodgers and Bigger, and McMillan, are poorly supported and difficult to reconcile with the treatment he later received as a prisoner of war in England. McMahon was commissioned a lieutenant in the Irish legion (16 June 1804) and served in the reserve at Mayence (1806) and then in Flanders (1807–9). Promoted captain, he was at Flushing when it fell to the British (16 August 1809). From then until the fall of Napoleon (April 1814) he was held as a prisoner of war in England, not, like other French officers, at large on parole but with private soldiers in a prison ship. In February 1810 he was an inmate of the Veteran at Gosport, from which he addressed a petition to the marchioness of Downshire, widow of his father's preferred candidate in the 1790 election, protesting, as a French citizen and officer, at his treatment. The petition did not succeed.
McMahon returned to France in May 1814, where he received five years’ arrears of pay and an order to join the 3ème régiment étranger. In the last of his letters to his sister, Jane Gordon, dated 7 February 1815, he wrote depreciatively of Napoleon: ‘if you consider him and the persons who conducted the French revolution to its end as Anti-Christ you will be very nearly right’. Hearing no more of him, his family believed he had fallen at Ligny or Waterloo, but the last mention of him is as a recipient of a military pension at Boulogne-sur-Mer in September 1816. As he was then said to be aged sixty-one and suffering from partial paralysis, he presumably died shortly afterwards. McMahon married Sophia Ashbourne or Ashburner, who, according to Kennedy, was ‘supposed to be a governess in the Londonderry family’ (Rodgers and Bigger); she bore him four children and went with them to her native England after his flight from Ireland. One of their two sons, Alexander, entered the East India Company army, lived to be ninety-four, and was father of the army general and geologist Charles McMahon (1830–1904) who demarcated the border between Persia, Afghanistan, and Baluchistan, the ‘McMahon line’.