Born in 1913 in Scotland. Was highly educated, attending Glasgow University and obtaining degrees in French and German at the Sorbonne in France before entering Cambridge University on scholarship to study modern languages.
Was introduced to Anthony Blunt and Guy Burgess and soon became a Communist working with the Cambridge Spy Ring. Monitored by Soviet agent Samuel Cahan, he received a short course in espionage tactics before taking the Home Office and Foreign Office exams, receiving the highest scores on both.
Was assigned to the Foreign Office in 1936 where he worked briefly with Donald Maclean. Served as Served briefly as the personal secretary to Lord Maurice Hankey who oversaw Intelligence services in Britain. Moved on to the Bletchley Code and Cipher School. In the course of his job he passed intercepted messages and other classified information to his Soviet handler. He often delivered cases full of intercepted German messages int he back seat of his car which he drove to the Soviet embassy.
During World War II, worked for MI6 in its London headquarters. Smuggled plans for postwar Yugoslavia to the Soviets. After World War II, continued to pass information to Soviet agents, including Kim Philby, Donald Maclean and Guy Burgess.
In 1951, sensitive documents in Cairncross’ handwriting were found in Burgess apartment after Burgess and Maclean fled to Russia. He was thus fired from his position in the British Treasury department, although he denied being a spy. He turned to scholarly activities and humanitarian efforts for the United Nations.
In 1964, Sir Anthony Blunt confessed to being a Soviet spy and in return for leniency identified Cairncross as another Soviet agent. When confronted with the evidence, Cairncross admitted to his espionage, explaining that he had not spied for several years, saying that he spied only during World War II, when Russia was a British ally.
Soviet defectors later disputed Cairncross statements about his limited involvement in espionage. They claimed that he had turned over countless reams of information.
Fearful of negative publicity and scandal, the British government hushed up his activities, declining to prosecute him for espionage or to expose him to the public. Cairncross, in fact, remained for a time in his job as with the United Nations Food and Agricultural Organization.
Cairncross was exposed in 1981 by Prime Minister Margaret Thatcher. He continued his life in exile until 1995 when he moved to Britain and married American opera singer Gayle Brinkerhoff. Later that year he died after suffering a stroke.