It was during his time at Cambridge that Cairncross was recruited to be a spy for the Soviet Union. Although Cairncross was a communist sympathizer, he could not afford to be identified as a member of the party due to the area of employment he desired to enter.

While at Cambridge University he attended communist meetings but he claimed he never actually joined the Communist Party of Great Britain. According to Michael Straight, he was already a Soviet mole and "was aiming for the civil service and could not therefore afford to be labeled as an open party member." Cairncross passed top of his year in the civil service examinations and was appointed to the Foreign Office on 14th October 1936. It is claimed that "his touchy and graceless manners made him unsuitable for a foreign posting." Arnold Deutsch was the head of recruitment for NKVD agents based in England. His network at this time included Kim Philby, Donald Maclean, Anthony Blunt and Guy Burgess. Deutsch's approached James Klugmann, another Soviet agent, who knew Cairncross when they were both at Trinity College. Klugmann now recruited Cairncross as a spy. His codename was LISZT. "His seething hatred of the British establishment was the impetus to treachery. His earlier failure to join the Communist Party was a bonus. In perfect tradecraft, Klugmann did not mention to his new recruit the names of others who were helping the Soviet cause." Cairncross later recalled how his Glaswegian accent was of some concern to Deutsch who felt that he needed to improve his diction if he was ever to get into the top echelons of Whitehall. (Simkin, 1997)

Fortunately for Cairncross, not officially registering as a member of the communist party proved to be an asset to his work as a spy, as did his place of employment. On March 10th, 1939, he was able to leak documents from the Foreign Office to the NKVD (Narodny Kommisariat Vnutrennikh Del or the People's Commissariat for Internal Affairs) (Farlex, 1988-2014; Simkin, 1997). The NKVD, which was previously the Government Political Administration, or GPA, was a forerunner to the KGB and essentially functioned as the communist secret police of the Soviet Union (Farlex, 1988-2014; Simkin, 1997).

In August- September 1938, LISZT (Cairncross) worked in the special 'crisis' group of the Foreign Office and had free access to documents on Munich. When he heard about his transfer to the Treasury, he took these documents with him and passed them at once on to us. In September 1938, LISZT saw in the Foreign Office a report from a British agent in the USSR on the unpreparedness of the USSR to render military assistance to Czechoslovakia. Among those in the Foreign Office who advocated an agreement with Hitler were the Ambassador in Berlin, Henderson, and the Ambassador in Paris, Phipps (Simkin, 1997).

1940 marked the year that Cairncross was appointed the private secretary to Maurice Hankey, the chancellor of the duchy of Lancaster. Hankey also served as the chairman to the Scientific Advisory Committee, which was developed to, "...coordinate the application of science to the war effort (Mellon, 2002, 12)." Since the committee dealt with scientific applications, they were also directly involved with the construction of atomic bombs and the research thereof. This allowed Cairncross to pass information about, "...the development of uranium atomic energy to produce explosive material" onto Gorsky, his controller, essentially initiating the first instance of any warning to the Soviet Union that the English and Americans were attempting to create an atomic bomb (Mellon, 2002, 12).