Pakenham, William Lygon (1819–87), 4th earl of Longford , army officer, and politician, was born 31 January 1819 at Pakenham Hall, Co. Westmeath, second son of Thomas Pakenham, 2nd earl of Longford, and his wife Georgiana Emma Charlotte (d. 12 February 1880), fifth daughter of William Lygon, 1st earl of Beauchamp. Educated at Winchester, he matriculated (May 1836) for Oriel College, Oxford, but left without taking a degree. He entered the army as an ensign in the 52nd Foot (25 August 1837) and, transferring to the 7th Foot, became a lieutenant (31 August 1838). In January 1844 he was promoted to captain, and in July 1852 to major. After the outbreak of the Crimean war (March 1854) he was promoted to lieutenant-colonel (12 December 1854) and appointed assistant quartermaster-general to the British expeditionary force in the east. Present at the battles of Alma, Balaclava, and Inkerman and the siege of Sevastapol, he was appointed assistant adjutant-general in late 1854. In June 1855 he was appointed adjutant-general and created CB (5 July 1855). On 17 July 1855 he advanced to full colonel and was later awarded several foreign decorations for his service in the Crimea; they included the Légion d'honneur (France), the Order of St Maurice and St Lazarus (Sardinia), and the Order of the Medjidie (Turkey).
During the Indian mutiny, he served as deputy adjutant-general to the forces in Bengal (1857–8), and later as adjutant-general (February 1858–July 1860). On the death (March 1860) of his elder brother, Edward Michael, 3rd earl of Longford, he succeeded to the earldom. In June 1861 he was made a KCB, and later served as under-secretary of state for war (July 1866–December 1868) in the administrations of the earl of Derby and Benjamin Disraeli. Promoted to major-general in 1868, he was awarded the colonelcy of the 5th Foot (1871), and in March 1874 appointed lord lieutenant of Co. Longford. He also served as DL and JP for Co. Westmeath. He became lieutenant-general in 1877, was made a full general in July 1879 and retired from the army in 1881. In May 1881 he was made a GCB.
Conservative in politics and a staunch unionist, he was an organiser of the Irish Loyal and Patriotic Union in 1885. Despite his opposition to home rule, he was popular with his tenants, and in his later years spent much time on his Co. Westmeath estate. He died of pneumonia, 19 April 1887, at 24 Bruton St., west London, and was buried in Kensal Green cemetery.
He married (November 1862) the Hon. Selina Rice-Trevor, fourth daughter of George, 4th Lord Dynevor; they had three sons and two daughters. In October 1864 his wife gave birth to twin sons, the older of whom, William Pakenham, Lord Silchester, died in February 1876. The other twin, the Hon. Thomas Pakenham, succeeded as 5th earl of Longford in 1887, served in the second Boer war, and was killed in action (21 August 1915) during the Gallipoli campaign.