He was born in Cambridge, and named after Rupert Brooke, who was a friend of his parents, but preferred to use his second name. He was educated at Stowe School and Trinity College, Cambridge. As an undergraduate, reading history, he joined the Communist Party of Great Britain. He was two or three years younger than the group of Trinity College communists including Guy Burgess, Donald Maclean, Kim Philby and James Klugmann.

Another Cambridge student, who would play a major part in his life, was Margot Heinemann. They were lovers, and he addressed both poems and letters to her. He also had a relationship with a Welsh woman, Rachel (Ray) Peters, with whom he had a child: James Cornford, who was subsequently adopted and brought up by his grandparents, Francis and Frances Cornford. A photograph of Peters and Cornford can be found at the National Portrait Gallery, London.

From 1933 he was directly involved in Communist Party work, in London, and became associated with Harry Pollitt. During the Spanish Civil War he both recruited in Cambridge for the International Brigade, and fought in the war himself, inspired in part by the Germans in the Thälmann Battalion.[1] He saw action at first in a Workers' Party of Marxist Unification (POUM) unit in Aragon in August 1936, before returning home, and coming back in December. He was killed at Lopera, near Córdoba.[2]

His brother Christopher continued to be active in politics until well into the 1980s.
In poetic terms, he was no modernist; as George Orwell pointed out in 1940, he represented continuity with the older, imperial tradition.