Aylmer, William (1778–1820), United Irish commander and officer in the Austrian army, was born in March 1778 at Painstown, Co. Kildare, the second son of Charles Aylmer (1720?–1801) but the first son of his second marriage, to Esmay, daughter of William Piers of Castletown, Co. Meath, and his wife Eleanor (née Dowdall); a younger son was Charles Aylmer (qv). William was commissioned as a lieutenant in the Kildare militia (5 August 1794) but, with several fellow officers, followed the example of their colonel, the 2nd duke of Leinster (qv), and resigned for political reasons in May 1797. Apparently while at camp at Loughlinstown, Co. Dublin, he had made the acquaintance of William Sampson (qv), a leading radical, and had fallen under his influence. When the United Irish rebellion broke out in the summer of 1798 Aylmer was in the rebel camp at Timahoe in the Bog of Allen (in north Kildare) with the rank of colonel and at the head of 3,000 rebels, 500 of whom attacked Maynooth on 10 June; he showed skill at guerrilla warfare, holding off attacks by government forces for some weeks. Early in July, accepting that the rebellion was defeated, Aylmer sent a message to his father, an acquaintance of the marquess of Buckingham, to try and arrange surrender terms for himself, his second-in-command, Hugh Ware (qv), and their remaining 300 followers. Detained in Dublin castle before being allowed to go to England, he was named in the Banishment Act (6 October).
In 1800 Aylmer went to Hamburg and on to Austria where, being an able horseman, he enlisted as a cadet in the 5th light dragoons of the imperial army (16 March); he was promoted sub-lieutenant (28 October 1800) and lieutenant (August 1804) in the 10th (later renumbered 6th) cuirassiers. Between 1800 and 1806 he was frequently absent from his unit. It has been suggested that William Wickham (qv), under-secretary at the Home Office when Aylmer was in England and afterwards British commissary in Austria, was employing Aylmer in some way (R. J. Aylmer, ‘Imperial service’, 216). At the great cavalry battle of Eckmühl (22 April 1809) Aylmer was left for dead but was captured by the French and later exchanged. He was made a full captain in 1813. After Napoleon's defeat, Aylmer was ordered to accompany Metternich's political delegation to London (21 May 1814). Despite his banishment he was received by the prince regent and also tutored British army officers in the art of swordmanship. He never returned to Austria but went back to Ireland, where he resided with his eldest brother, Robert (1770?–1841), at Painstown. He was not finally discharged from the imperial service until February 1816.
A few years later, perhaps restless from being inactive, he answered a request from John Devereux (qv) to raise and command a lancer regiment for the Irish legion that was supporting Bolívar's rebellion against Spanish rule in South America. On 25 September 1819 he landed on Margarita off the Venezuelan coast. He was appointed temporary commander of the Irish legion on the island, but he found it hard to keep discipline. In 1820 he crossed over to the mainland to fight at the battle of Río Hacha, where he suffered wounds that did not heal. With mutineers and other wounded, Aylmer was shipped to Jamaica, where he died 20 June 1820. He never married.