Born in Northumberland, both of his parents were Members of Parliament for the Liberal Party. His father was created Viscount Runciman of Doxford in 1937. His paternal grandfather, Lord Runciman, was a shipping magnate.
He was named after his maternal grandfather, James Cochran Stevenson, the MP for South Shields.
It is said that he was reading Latin and Greek by age five. In the course of his long life he would master an astonishing number of languages, so that, for example, when writing about the Middle East, he relied not only on accounts in Latin and Greek and the Western vernaculars, but consulted Arabic, Turkish, Persian, Hebrew, Syriac, Armenian and Georgian sources as well. A King's Scholar at Eton College, he was an exact contemporary and close friend of George Orwell. While there, they both studied French under Aldous Huxley. In 1921 he entered Trinity College, Cambridge as a history scholar and studied under J.B. Bury, becoming, as Runciman later commented, "his first, and only, student." At first the reclusive Bury tried to brush him off; then, when Runciman mentioned that he could read Russian, Bury gave him a stack of Bulgarian articles to edit, and so their relationship began. His work on the Byzantine Empire earned him a fellowship at Trinity in 1927.
After receiving a large inheritance from his grandfather, Runciman resigned his fellowship in 1938 and began travelling widely. From 1942 to 1945 he was Professor of Byzantine Art and History at Istanbul University, in Turkey, where he began the research on the Crusades which would lead to his best known work, the History of the Crusades (three volumes appearing in 1951, 1952, and 1954). Most of Runciman's historical works deal with Byzantium and her medieval neighbours between Sicily and Syria; one exception is The White Rajahs, published in 1960, which tells the story of Sarawak, an independent nation founded on the northern coast of Borneo in 1841 by the Englishman James Brooke, and ruled by the Brooke family for more than a century.
In his personal life, Runciman was an old-fashioned English eccentric, known, among other things, as an aesthete, raconteur, and enthusiast of the occult. According to Andrew Robinson, a history teacher at Eton, 'he played piano duets with the last King of China, told tarot cards for King Fuad of Egypt, narrowly missed being blown up by the Germans in the Pera Palace hotel in Istanbul and twice hit the jackpot on slot machines in Las Vegas'. In these respects, he was not a typical Medieval historian. He was also known for his remarkably sunny disposition and openness of spirit, and had friends from all walks of life and classes in many countries. He died in Radway, Warwickshire.