His father was professor of ancient philosophy at Cambridge University and his mother, the grand-daughter of Charles Darwin, was a published poet. He was named Rupert in memory of Rupert Brooke, who had been killed in the First World War just before he was born.
At the age of nine Cornford was sent as a boarder to Copthorne Preparatory School in Sussex. Francis Macdonald Cornford later recalled: "At Copthorne he became absorbed in cricket. His headmaster said that his style, as a player, was the most incredible he had ever seen. But his knowledge of the history of the game was exhaustive... Later, his knowledge of the more democratic Association Football became no less extensive."
In 1929 he obtained a scholarship to Stowe School. The following year he was joined by his brother, Christopher: "Already, by the time I joined him at Stowe in the autumn of 1930, he had begun to be critical of the school, as indeed of everything else. He was already anti-militarist and atheist - one of his favourite pastimes was to tie up the school chaplain in metaphysical knots during the Tuesday afternoon religious talks."
The following year he began writing poetry that had been inspired by the work of W.H. Auden, T.S. Eliot and Robert Graves. He also developed an interest in politics. Christopher Cornford recalled: "As young as fourteen and a half he became sympathetic to Socialism. As we strode together through the school grounds, among the great beech trees and lakes, the rotundas and monumental obelisks, in shiny blue serge Sunday suits and stiff collars unloosed, he explained to me the principles of the nationalisation of industry and the injustices of our economic system."
Cornford also began reading books written by G. D. H. Cole, Harold Laski, Rajani Palme Dutt, Karl Marx and Friedrich Engels. He claimed that Cole's plan for socialism "isn't powerful enough emotional position to accumulate the colossal reserves of energy and fanaticism that are needed to bring through a revolution without violence". Cornford preferred Marx to Cole: "Cole's Socialism is without the strength of Communism because it is without the really important part of Marxism, his dialectical materialism and the interpretation of history." He described Palme Dutt as "extraordinarily intelligent, but almost equally bitter."
In 1932 he read Das Kapital and the Communist Manifesto. He also wrote a letter to his brother about the criticism that Karl Marx had received from Harold Laski. " It seems to me dishonest for men like Laski to dismiss the Marxist interpretation of history and yet proclaim Marx as a great prophet, because his wonderfully accurate prophecy is dependent on his interpretation of history. Where it seems to me that he went wrong is in applying terms like the class-struggle (which is a legitimate abbreviation of what actually happens) as the whole and simple truth. It's far more complicated than he seemed to realise. But I believe that in this, too, his limitations are important in making him intelligible."
When he was only sixteen he won an exhibition to Trinity College. Cornford spent that summer in London. He joined the Young Communist League and spent a lot of time at the London School of Economics. He also had a poem published in the Listener before taking up his place at university in October 1933. According to Michael De-la-Noy: "During his three years at Cambridge he wrote only nine poems, for he was spending fourteen hours a day on political activities."
A fellow student at university was Victor Kiernan. He later recalled: "He had his philosophy, or rather instinctive attitude, of which friends got only glimpses; not that lie was secretive, but that he followed Lenin in his contempt for all useless sentiment and psychological weakness. He recalled one's ideas of what Lenin was like in other ways. His politics, unlike those of many middle-class Socialists, were not based on humanitarianism alone. To him the movement was something that could call out and realise all his powers; it was the only atmosphere he could breathe."
Cornford became very concerned about Adolf Hitler gaining power in Nazi Germany. In an article that he wrote in The Cambridge Left journal in 1934 he argued: "Fascism strives to cripple the working-class movement by murdering and torturing its leaders, suppressing its legal organisations and press, removing the right to strike in defence of wages and conditions, and all political rights whatsoever. Fascism exploits the Nationalist feelings of the petty bourgeoisie to divert their hostility towards the existing regime by whipping up a chauvinist frenzy against some foreign scapegoat - in Germany the Jews; in Poland the Ukrainian minority."
In March 1935 Cornford became a full member of the Communist Party of Great Britain. Despite the amount of time he spent on his political activities he achieved first classes in both parts of the history tripos (1935 and 1936). Cornford also became romantically attached to Rachel Peters and she gave birth to his illegitimate child. He later left Peters for Margot Heinemann.
Kenneth Sinclair Loutit recalled how John Cornford and James Klugman tried to recruit him into the Socialist Society: "My second meeting with James Klugman must have been after my return from Germany. He was accompanied by John Cornford. As contemporaries we all knew each other by sight, and Klugman remembered his previous recruiting visit. They said, very reasonably, that it was only by working together that people sharing the same goals could hope to achieve them, so I really could not do other than join the University Socialist Society. I had already told James the year before that I was not a joiner... John Cornford then took over asking what I saw as the most important thing for the next decade."
John Cornford wrote an article explaining why so many university students were joining the Communist Party of Great Britain. "The last few years have seen a considerable growth of Communist influence in the universities... It is no longer a phenomenon that can be dismissed as an outburst of transient youthful enthusiasm. It has established itself so firmly that any serious analysis of trends in the universities must take it into account... Communism in the universities is a serious force. It is serious because students do not easily or naturally become Communists. Communism has to fight down more prejudices, more traditions, more simple distortions of fact, than any other political organisation. It would not have gained ground without a serious appeal."
In 1936 Trinity College gave him a scholarship, and he planned to study the Elizabethans. However, on the outbreak of the Spanish Civil War he decided to go to Spain. Cornford reached Barcelona on 8th August 1936. He wrote to Margot Heinemann about the revolutionary spirit he found in Spain: "In Barcelona one can understand physically what the dictatorship of the proletariat means.... The mass of the people ... simply are enjoying their freedom. The streets are crowded all day, and there are big crowds round the radio palaces. But there is nothing at all like tension or hysteria... It is genuinely a dictatorship of the majority, supported by the over-whelming majority.
He met Franz Borkenau, an Austrian journalist, and the two men decided to travel to the front-line. He had already made up his mind to join the Worker's Party (POUM) army. He told his father in a letter: "After I had been three days in Barcelona it was clear, first, how serious the position was; second, that a journalist without a word of Spanish was just useless. I decided to join the militia."
Tom Wintringham later explained why Cornford had joined a group that was strongly influenced by the political ideas of Leon Trotsky and hostile to Joseph Stalin and his government in the Soviet Union: "In Barcelona he (John Cornford) found that the militia was being organized by the trade unions and political parties. He had no papers from England: he had not even brought his party membership card with him. So when he applied to the Hotel Colon, then the military headquarters of the party in which Barcelona's Socialists and Communists had joined forces, he was told to wait. Friendly but precise Germans, people taught by Bismarck as well as by exile to carry all necessary documents with them at all times, told him that he could not join the Thaelmann group, or any other unit of foreign volunteers organized by the United Socialist Party, until his standing as a known anti-Fascist was guaranteed by some document or by some person they knew. John was too restless, impatient for this. He flung off to join a rival party's militia, that of the P.O.U.M."
Cornford took part in an action at Perdiguera. He wrote to Harry Pollitt about his whereabouts and to inform him of the realities of the military situation in Spain. He also told Margot Heinemann about the battle he had taken part in and how it was reported in the POUM newspaper: "Today I found with interest but not surprise the distortions in the POUM press. The fiasco of the attack at Perdiguera is presented as a punitive expedition which was a success."
Cornford fought at Aragon in August 1936. The following month he fell ill and was sent to hospital. The authors of Journey to the Frontier have argued: "The precise nature of John's illness has never been established... it was disabling, and he suffered a few truly difficult days." Cornford was sent back to Cambridge to recuperate.
Cornford decided to use this as an opportunity to persuade some of his old friends to go to Spain. Bernard Knox later recalled: "In September I received a letter from my friend John Cornford, the leader of the Communist movement in Cambridge, who had just returned from Spain, where he had fought for a few weeks on the Aragon front, in a column organized by the Partido Obrero de Unificación Marxista, the POUM, a party that was later to be suppressed as too revolutionary. He had returned to England to recruit a small British unit that would set an example of training and discipline (and shaving) to the anarchistic militias operating out of Barcelona. He asked me to join and I did so without a second thought."
Cornford recruited a group of twelve friends to join the International Brigades. Knox went with him to see his father, Francis Macdonald Cornford. "He had served as an officer in the Great War and still had the pistol he had had to buy when he equipped himself for France. He gave it to John, and I had to smuggle it through French Customs at Dieppe, for John's passport showed entry and exit stamps from Port-Bou and his bags were likely to be given a thorough going-over."
Cornford took the men to Albacete, where they received some military training. Bernard Knox later recalled: "Our British section was assigned (mainly, I suppose, because I could serve as interpreter) to the French Battalion, where we ended up in the compagnie mitrailleuse, the machine-gun company. But for the rest of September and all through October we had no machine guns, not even rifles; the only weapon around was John's pistol, which he kept well under wraps. Since we couldn't train with weapons, our days were spent practicing close-order drill (French, English, or sometimes Spanish) and going on route marches along the dusty roads of the province of Murcia. No one knew when or where we would be sent to fight when (if ever) the weapons arrived, though the scuttlebutt rumors had us held in reserve for a flanking movement via Ciudad Real that would take Franco, now moving steadily toward Madrid, in the rear."
Bernard Knox fought alongside Cornford in Spain. "Our baptism of fire was sharp and unexpected. We were scattered with our machine-guns along a crest which we had every reason to believe was as safe as anything could be in the Madrid area (which wasn't very safe), when we heard our first shell. Nobody minded much, because it burst a good forty yards behind us, but the next two or three showed us that they were feeling for the crest we were occupying... Our commander had gone up to advanced positions that night with one of our gun-crews, so John took over command that morning, inspecting the positions we had taken up, and criticising ruefully the way in which most of us came down the cliff. But it was not a bad performance for raw troops taken by surprise in a barrage."
Cornford took part in the battle for Madrid and on 7th November 1936, and received a severe head wound. Sam Russell was with him when it happened: "When the smoke cleared there was John Cornford with blood pouring down his face and head. We later discovered that it was one of our own anti-aircraft shells that had fallen short and had come through the side of a wall. They took John off and that afternoon he came back with his head bandaged, looking very heroic and romantic."
While recovering he wrote some of his most important poems including Heart of the Heartless World. On 8th December, 1936, Cornford wrote to Margot Heinemann: " No wars are nice, and even a revolutionary war is ugly enough. But I'm becoming a good soldier, longish endurance and a capacity for living in the present and enjoying all that can be enjoyed. There's a tough time ahead but I've plenty of strength left for it. Well, one day the war will end - I'd give it till June or July, and then if I'm alive I'm coming back to you. I think about you often, but there's nothing I can do but say again, be happy, darling, And I'll see you again one day."
John Cornford insisted on going back to the front-line where he joined the recently formed British Battalion. He was killed near Lopera on 27th December 1936, his twenty-first birthday. According to Francis Beckett, the author of Enemy Within - the Rise and Fall of the British Communist Party (1995), Cornford was killed trying to retrieve the body of Ralph Fox. Cornford's best known work was published after his death: The Last Mile to Huesca and Poems from Spain.