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.