Vandeleur, Sir John Ormsby (1763–1849), British army general, was eldest son of Richard Vandeleur, captain in the 9th Lancers, of Rutland, Queen's Co. (Laois), and Kilrush, Co. Clare, and his wife Elinor, daughter of John Firman of Firmount. Educated privately, he joined the army in December 1781 and was commissioned as an ensign in the 5th Foot. Promoted to lieutenant (1783), he transferred into the 67th Foot and served in the West Indies. Transferring into the 9th Foot (1788), he was promoted to captain (March 1792) and changed regiments once again in the same year when he joined the 8th King's Royal Irish Light Dragoons. This proved to be a turning point in his career and, promoted to major (March 1794), he would later distinguish himself as a cavalry officer of dash and ability.
He served with the duke of York in the Flanders campaign of 1794–5 before travelling to South Africa, where he served with Gen. Craig and Gen. Dundas in the campaign against the Dutch settlers in the Cape. Promoted to lieutenant-colonel (January 1798), he published Duty of officers commanding detachments (1801). Posted to India (1802), he played a significant part in the second Mahratta war (1803–5). Given the local rank of colonel, he took command of a brigade of light cavalry and, at the decisive battle of Leswaree/Laswaree (1 November 1803), turned the left flank of the Mahratta army and took over 2,000 prisoners. On 17 November 1804 at Farrukhabad he attacked the camp of the Mahratta chief Jaswant Rao Holkar, routing his cavalry. At the battle of Afzalghar (2 March 1805) he led a daring charge and recaptured British guns that had been taken by Mahratta forces. Throughout the campaign he was praised by his commanding officer, Lord Lake (qv), who frequently mentioned Vandeleur in his despatches.
Returning to England, he transferred into the 16th Light Dragoons (April 1807) and was promoted to brevet-colonel (April 1808). In June 1811 he was promoted to major-general and appointed to command a light infantry brigade in the Peninsula. This posting to an infantry command was not to his taste, but he proved himself an able commander during the first phase of his Peninsular career. During the storming of Ciudad Rodrigo (19 January 1812) the light division's commander, Gen. Robert Craufurd, was mortally wounded and Vandeleur took command of the division, leading it into the breach in the city's walls. Wounded in the shoulder in the attack, he was mentioned in the Ciudad Rodrigo despatch by the duke of Wellington (qv). He took part in the battle of Salamanca (22 July 1812), and a few days before the battle of Vittoria (21 June 1813) his division surprised and scattered a French brigade, taking over 300 prisoners. He fought at Vittoria and, appointed to command a brigade of light dragoons, took part in all the major actions during Wellington's march into the south of France. Present at the inconclusive battle of Nive (9–12 December 1813), he remained with the occupying troops in France after Napoleon's abdication in 1814. He was awarded the gold Peninsular war medal with four clasps. In October 1814 he was appointed to Wellington's staff in Belgium and was created a KCB on 3 January 1815, being made honorary colonel of the 19th Light Dragoons on 12 January.
Renowned for his coolness under fire, he achieved prominence during the Hundred Days campaign of 1815. As Napoleon's army marched on Belgium, Vandeleur was appointed to command a brigade of light cavalry consisting of the 11th, 12th, and 16th Light Dragoons. On the morning of the battle of Waterloo (18 June 1815), he positioned his brigade on the left of the allied line and, during the course of the battle, led a number of timely and well executed charges. After the charge of the Household and Union brigades, he led his cavalry forward to cover their disorganised retreat, protecting the remnants of the Union brigade from attacks by French cavalry led by Gen. Durutte. Wellington later ordered him up to support some divisions of Belgian–Dutch infantry that appeared to be about to give way in the face of remorseless French assaults. When the earl of Uxbridge was badly wounded and had to leave the field, Vandeleur, as the senior surviving cavalry commander, took over command of the allied cavalry, leading a series of charges against the flagging French infantry and cavalry. Mentioned in Wellington's Waterloo despatch, he was awarded the silver Waterloo medal.
In the aftermath of the Napoleonic wars, he received several honours including the Order of St Vladimir (Russia) and the Order of Maximilian Joseph (Bavaria). In July 1821 he was promoted to lieutenant-general, subsequently being promoted to full general (1838). He was awarded the honorary colonelcies of the 14th Light Dragoons (1823) and the 16th Lancers (1830), being made a GCB in 1833. Returning to Ireland, he lived in quiet retirement, dying on 1 November 1849 at his Dublin residence, 30 Merrion Square North.
He married (1829) Catherine, daughter of the Rev. John Glasse; they had one son and two daughters. Their younger daughter, Ellen Vandeleur, married Col. (later Gen.) Richard Greaves, a veteran of the British–American war and assistant military secretary to the commander-in-chief in Ireland for over twenty years. There is a small collection of Sir John Ormsby Vandeleur's letters in the library of TCD, in which there are several references to the Waterloo campaign.