Crampton, Sir John Fiennes Twisleton (1805–86), diplomat, was born in Dublin, elder son of Sir Philip Crampton (qv) of Merrion Square, Dublin, surgeon to the Meath Hospital and surgeon-general to the forces in Ireland. He was educated at Eton and TCD (1821). He entered the British diplomatic service in 1826 as an unpaid attaché, his first post being at Turin. In 1828 he was transferred to St. Petersburg and in 1834 became an attaché at Brussels. He held the same post at Vienna from 1839 until becoming secretary to the legation to the Confederated States of Swiss Cantons at Berne (1844). He was posted in July 1845 to Washington, where he served as chargé d'affaires from May 1847 to December 1849, and again from August 1850 to January 1852. He was then appointed minister plenipotentiary and envoy extraordinary to the United States of America.
During the Crimean war (1854–6) he was involved in the recruitment of soldiers in the USA for the British army. By 1856 this activity had aroused the strong disapproval of the American administration, and the secretary of state, William Marcy, demanded that Crampton and three British consuls be recalled, despite the apologies of Lord Clarendon (qv). According to Marcy, Crampton had continued to recruit even after it had been declared unlawful and only desisted when ordered by his government to do so. In May 1856 Crampton returned to England after President Franklin Pierce had broken off diplomatic relations with him. Though Lord Palmerston reinforced the Royal Navy units on the North American station, he accepted Crampton’s recall and outright hostilities were avoided. Crampton was made a KCB 20 September 1856. In March 1857 he was appointed minister plenipotentiary and envoy extraordinary at Hanover. In 1858 he was transferred to the embassy at St Petersburg before moving to Madrid in 1860, remaining there until his retirement in 1869.
He succeeded his father as 2nd baronet on 10 June 1858, and married (1860) Victorine (1837–71), daughter of the composer Michael William Balfe (qv). They were divorced three years later. He returned to live in Ireland at Bushey Park, Enniskerry, Co. Wicklow, where he died 5 December 1886.