James Cochran Stevenson ‘Steven’ Runciman was born on 7 July 1903 in Northumberland. He was the younger son of Walter Runciman, later to become the first Viscount Runciman of Doxford, and his parents were the first married couple to sit as MPs at the same time. He studied history at his mother's alma mater, Trinity College, Cambridge, taking a first class degree in 1925. Following his postgraduate studies under J. B. Bury, he became a fellow of the college in 1927, and a lecturer in 1932.

Following the death of his grandfather in 1937 and his inheritance of a rather substantial sum, in 1938, he resigned his lectureship in order to focus fully on writing. He served in the Second World War as a press attaché in Sofia and a film censor in Cairo and Jerusalem before being appointed professor of Byzantine art and history at the University of Istanbul, a post he held until 1945. This would be his last academic post, save for an honorary fellowship at Trinity College, Cambridge, in 1965.

Retirement from a permanent academic post did not stop Runciman from participating actively in academic life. In addition to countless smaller lectures, he delivered the Waynflete lectures at Oxford in 1953–1954, the Gifford Lectures at the University of St Andrews between 1960 and 1962, and the Birbeck lectures at Trinity College, Cambridge, in 1966. Due in large part to his work on the history of the Orthodox Church, he was named the Grand Orator of the Greek Church, the senior lay position in the patriarch’s synod.

In addition to various academic honours, he was knighted in 1958 and appointed a Companion of Honour in 1984. He never married, and died on 1 November 2000.

His publications include: The Emperor Romanus Lecapenus (1929); The First Bulgarian Empire (1930); Byzantine Civilization (1933); The Medieval Manichee (1947); History of the Crusades (vol. 1, 1951; vol. 2, 1952; vol. 3, 1954); The Sicilian Vespers (1958); The White Rajahs (1960); The Fall of Constantinople (1965); The Great Church in Captivity (1968); The Last Byzantine Renaissance (1970); The Orthodox Churches and the Secular State (1972); The Byzantine Style and Civilisation (1975); A Traveller's Alphabet (1991).