Cairncross's father was the manager of an ironmonger's and his mother a primary school teacher. John Cairncoss was one of a family of eight, many of whom had distinguished careers. All three of his brothers became professors. One was the economist Sir Alexander Kirkland Cairncross (a.k.a. Alec Cairncross). The journalist Frances Cairncross is his niece. Cairncross grew up in Lesmahagow, a small town on the edge of moorland, near Lanark in the Central Belt of Scotland, and was educated at the Hamilton Academy (although his name appears as the 1928 winner of the Dux prize at Lesmahagow High School); the University of Glasgow; the Sorbonne and Trinity College, Cambridge, where he studied French and German.
After graduating, he took the British Civil Service exam and won first place. An article in the Glasgow Herald on 29 September 1936 noted that Cairncross had scored an "outstanding double success of being placed 1st in the Home List and 1st in the competition for the Foreign Office and the Diplomatic Service," and that he had been placed 5th in the (Glasgow University) bursary competition of 1930, and was also a Scholar and Bell Exhibitioner at Trinity College, Cambridge.
Cairncross worked initially in the Cabinet Office as a private secretary to Lord Hankey, the Chancellor of the Duchy of Lancaster. Later he transferred to the Foreign Office. It has been suggested that in 1937 he joined the Communist Party of Great Britain, but he was not noted whilst at Cambridge for any political activity. He was regarded as rather austere and uncommunicative as an undergraduate.
In 1942 and 1943 Cairncross worked in GC&CS, Bletchley Park on ULTRA ciphers. In 1944, he joined MI6.
During his time at Bletchley Park, Cairncross passed documents through secret channels to the Soviet Union.
Cairncross, codenamed Liszt by the Russians because of his love of music, had been instructed to get into Bletchley Park, known to the KGB as Kurort.
From 1942 onwards, the German High Command communicated with Army group commanders in the field using a machine that the British codenamed Tunny. Colossus, the world's first electronic digital computer, deciphered Tunny messages in quantity from 1943 onwards. Cairncross smuggled Tunny decrypts due to be destroyed out of Hut 3 in his trousers, transferring them to his bag at the railway station before going to meet his KGB contact in London.
The raw transcripts decrypted by Colossus were passed to intelligence officers at Bletchley Park, who created reports based on this material by disguising its origin as signals traffic. By providing verbatim transcripts, Cairncross showed the Soviets that the British were breaking German codes.
It was at that time considered to be in the British interest for the Soviet Union to be made aware of German military plans, but not of how they were obtained. Only information based on these reports was passed to the Russians through official channels. However, Stalin distrusted unsourced intelligence presented to him by Britain and the United States.
Operation Citadel was the codename given by Nazi Germany to their offensive which led to the Battle of Kursk, a turning point on the Eastern Front. After being defeated at Kursk, the Wehrmacht retreated steadily until Berlin was taken.
Tunny decrypts (transcripts) gave the British advance intelligence about Operation Citadel whilst it was being planned. Almost all raw transcripts were destroyed at the end of the war but a surviving transcript dated 25 April 1943 from German Army Group South signed by Maximilian von Weichs shows the high level of detail available to British intelligence officers. Analysts deduced the northern and southern attack routes, and a report based on this transcript was passed through official channels to Stalin.
During this period, Cairncross provided a second clandestine channel, supplying raw Tunny transcripts directly.
Axis occupation forces in Yugoslavia used radio communication extensively. In addition to German Abwehr, Sicherheitsdienst, Luftwaffe, Naval, railway, Army group and High Command messages, GC&CS intercepted and decrypted Yugoslav partisan communications with Comintern and with the Soviet Union. Cairncross first in Hut 3, then later at MI6 HQ, had access to raw decrypts. Communications from Comintern to Tito supplying some of this intelligence strongly suggests that he passed decrypts concerning Yugoslavia to the KGB.
Between 1941 and 1945, Cairncross supplied the Soviets with 5,832 documents, according to Russian archives. In 1944, Cairncross joined MI6, the foreign intelligence service. In Section V, the counter-intelligence section, Cairncross produced under the direction of Kim Philby an order of battle of the SS. Later Cairncross would suggest that he was unaware of Philby's connections with the Russians.
Yuri Modin, the Russian MGB (later KGB) control in London claims that Cairncross gave him details of nuclear arms to be stationed with NATO in West Germany. He gives no date for this message. But Cairncross was at the Ministry of Supply in 1951 and NATO was established in April 1949. However, there was no such plan at this time and it was only much later that NATO obtained tactical nuclear weapons under US control in Germany. This appears to have been a disinformation exercise.
Cairncross admitted to spying in 1951 after MI5 found papers in Guy Burgess's flat with a handwritten note from him, after Burgess's flight to Moscow. Some believe that he may have supplied information about the Western atomic weapons programme, the Manhattan Project, to assist the Soviet nuclear programme. It would have been surprising, though, if he had clearance to any useful engineering information, or that he would have understood it. He was never prosecuted, however, which later led to charges that the government engaged in a conspiracy to cover up his role. Indeed, the identity of the infamous 'fifth man' in the Cambridge Five remained a mystery outside intelligence circles until 1990, when KGB defector Oleg Gordievsky confirmed Cairncross publicly. Cairncross actually worked independently of the other four and did not share their upper-middle-class backgrounds or tastes. Although he knew Anthony Blunt at Cambridge and Guy Burgess in the Foreign Office (and had a personal dislike of both of them), he claimed not to have been aware that they or any of the others were also passing secrets to the Russians.
At the end of the war Cairncross joined the Treasury - he claimed that he ceased working for the MGB (later to become the KGB), at this time. KGB reports, published subsequently, contradict this.
After his first confession (1952), Cairncross lost his civil-service job and was penniless and unemployed. He moved to the United States as a lecturer at Northwestern University and at Case Western Reserve University. He became an expert on French authors and translated the works of many 17th century French poets and dramatists such as Jean Racine, Jean de La Fontaine and Pierre Corneille, as well as writing three of his own books: Molière bourgeois et libertin; New Light on Molière; and After Polygamy was Made a Sin.
This career was ended following further investigation into Cairncross by MI5 investigative officer Arthur S. Martin. After Philby fled to Moscow in 1963, Martin reopened the files to hunt for the fourth and fifth men. To Martin's surprise, Cairncross made a full confession. Martin also received a denunciation which led to Blunt's confession.
Cairncross moved to Rome, where he worked for the United Nations Food and Agriculture Organization as a translator, also taking on work for the Research Office of Banca Nazionale del Lavoro, Banca d'Italia and IMI. In the BNL, a young economist engaged with international scenarios analysis (the Iraq-Iran War of 1980-1988, petroleum's strategic routes in the Middle East and Far East) reported a strong and unusual interest by Cairncross about the Bank's role in that area. During his time in Rome his secret finally reached the public. In December 1979, Barrie Penrose, a journalist, concluded that Cairncross was the ‘fifth man’ and confronted him. Cairncross's third confession became front-page news. His status as the ‘fifth man’ was supported years later by Oleg Gordievsky, the KGB defector. Cairncross retired to the south of France until 1995 when he returned to Britain and married American opera singer Gayle Brinkerhoff. Later that year he died after suffering a stroke, at the age of 82.
Cairncross's autobiography, The Enigma Spy, was published in 1997. In 2001 writer Rupert Allason lost a court case in which he claimed to have ghostwritten The Enigma Spy in return for copyright and 50% of the book proceeds.
Cairncross appears as a character in the Franco-Belgian comic India Dreams by Maryse Charles and Jean-François Charles. He was also depicted in part three of the 2003 BBC TV series Cambridge Spies, where he appears reluctant to continue passing Bletchley Park data to the Russians for fear that the Red Army was heavily penetrated by German intelligence and by Eastern Front military intelligence under General Gehlen; Anthony Blunt is depicted in the drama as pressuring him with threats to continue. He is depicted as the fifth of the Cambridge Five in Frederick Forsyth's The Deceiver.
Cairncross appears as a cryptographer at Bletchley Park in the 2014 film The Imitation Game, played by Allen Leech. He is shown as an unwitting double agent being used as a back-channel by MI6 to pass information to the Soviets that Churchill is too cautious to provide; no historical basis for this is provided.