He was educated at Eton, and joined the Argyll and Sutherland Highlanders in 1939 when the Second World War broke out. An amateur radio ham, he was quickly transferred to the Royal Corps of Signals, and was commissioned, ending up as a Captain. He read Natural Sciences at New College, Oxford, from 1940 to 1942, before returning to Catterick to train for the invasion of France. He landed with his brigade in Normandy three weeks after D-Day, and took charge of a team of signallers. After the war, he chanced to meet Sir Archibald Clerk Kerr, a family friend, on a bus in London. Kerr had been ambassador in Moscow during the war, and had just been appointed British ambassador to the United States; he asked Anstruther to become his private secretary. Anstruther readily agreed, and spent four years in America in the Diplomatic Service. He moved to Paris in 1951, to advance his ambition to become a writer. He met Geraldine Horner, elder daughter of Captain Gerald Stuart Blake, and they were married on 7 March 1951. They had one daughter Emily who later married Simon Crosby. The baronet was divorced in 1963, and he married the architect Susan Margaret Walker daughter of Colonel St John Bradling Paten on 15 November 1963. They had two sons and three daughters. The eldest Sir Sebastian inherited the title and estates in 2007. In 1992 he married a Thai girl, Pornpan Pinthwong known as Goy. Their first child, Maximillian was born in 1995. They still live on the Barlavington estate. Sir Ian was surprised to inherit an estate in South Kensington from his aunt Joan in 1960, making him wealthy. He had bought a country estate at Barlavington, on the north of the South Downs near Petworth in West Sussex, in 1956, including of woodland, farmland and downland. He also bought a house near St. Tropez in 1973.
He enjoyed cars, owning an Aston Martin DB6, a Maserati, and several Porsches, but disliked excessive speed, and was occasionally stopped by the police for driving too slowly. He later traded down to a smart car. He succeeded his cousin Sir Ralph Anstruther, 7th Baronet, in 2002, inheriting two Anstruther Baronetcies – of Nova Scotia, of Balcaskie and of Anstruther. His cousin had been hereditary Carver to the Sovereign in Scotland, but the office passed instead to his second son, Toby. He also believed that he held the British baronetcy of Anstruther, but its remainder would have made it extinct on the death of Sir Windham Carmichael-Anstruther, 11th Baronet, in 1980, as most reference books, such as Burke and Debrett, have noted. As an adult, he adhered to a fixed routine. He habitually wore a bow tie in the day, and a cravat in the evening. He walked each day in the South Downs, lunching at one of five village pubs during the week, always drinking ginger beer. He took tea at 5pm, and ate supper 8.30pm. He always dressed for dinner in a velvet suit and silk cravat, before his two Martinis. His family knew that matters were serious when he failed to dress for dinner a few weeks before his death. He died at Barlavington. He was survived by his daughter from his first marriage, and two sons and three daughters from his second marriage. Due to differences between English law and Scottish law, one son, Sebastian, inherited the Scottish title, becoming 9th Baronet of Balcaskie and 11th Baronet of Anstruther. Obituaries to Sir Ian noted, erroneously, that the Great Britain Baronetcy of Anstruther had passed to his other son Toby. He left an estate valued in excess of £35,000,000.
