MacMahon, Hugh Óg (d. 1644), soldier and conspirator, was the eldest legitimate son of Sir Brian MacHugh Óg MacMahon of Dartree (Dartry), Co. Monaghan, by a daughter of Hugh O'Neill (qv), 2nd earl of Tyrone. Fostered until the age of 16 with the Connolly family of Killeevan, Co. Monaghan, he developed a close friendship with his foster-brother Owen, who subsequently converted to protestantism while in the service of Sir John Clotworthy (qv), an Antrim planter. According to his cousin Randall MacDonnell (qv), 2nd earl of Antrim, who interceded unsuccessfully with the lord deputy on his behalf, MacMahon was ‘oppressed by his elder and illegitimate brothers in his minority’ (quoted in Ohlmeyer, 55). He served as lieutenant-colonel in the Spanish army during the 1630s, but little is known of his time on the Continent. He returned to Ireland in 1640, inheriting an estate from his nephew near Clones in Monaghan, and served as a JP in the county.
Publicly insulted at the Monaghan assize by Edward Aldrich, sheriff of the county, MacMahon decided to seek out opponents of the regime in Dublin. He does not appear to have played any role in planning the events of October 1641, but his nephew Philip MacHugh O'Reilly (qv) included him in the party sent to capture Dublin castle. The night before the intended raid, MacMahon divulged the plotters' intentions to Owen Connolly in Dublin, who immediately betrayed the plot to the lords justices. He was seized by the authorities, along with Conor, Lord Maguire (qv), imprisoned in Dublin castle and tortured. Whereas Maguire denied any knowledge of the plot, MacMahon allegedly ‘confessed enough to destroy himself and impeach some others’ (DNB). Sent to England in June 1642 with Maguire and Col. John Read, he was committed to the Tower of London and examined by the judges of the king's bench. The trial failed to progress, however, due to the difficulty in obtaining witnesses from Ireland, where a full-scale revolt had erupted across the country. On 17 August 1644 both MacMahon and Maguire escaped from prison, with the assistance of two priests from the Spanish embassy in London. At large for almost five weeks, they were finally discovered, after an intensive manhunt, in a house on Drury Lane in London and returned to the Tower. MacMahon was brought to trial on 13 November 1644, found guilty of high treason, and executed nine days later. Although MacMahon was a relatively minor figure prior to the revolt, his inadvertent role in foiling the attack on Dublin castle, his dramatic escape with Lord Maguire from the Tower in London, and subsequent high-profile trial and execution, ensured him a certain historical prominence.