After the war, he chanced to meet Sir Archibald Clerk Kerr, a family friend, on a bus in London. Kerr (later 1st Baron Inverchapel) 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.