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.