Mitrokin was born in central Russia in 1922 and entered the MGB as a foreign intelligence officer in 1948. His first foreign posting was in 1952. Between 1972 and 1984 he supervised the move of the archive of the First Chief Directorate from the Lubyanka to the new KGB headquarters at Yasenevo. He retired in 1985.
In 1992 he traveled to Latvia with copies of material from the archive and walked into the US embassy. CIA officers there didn't consider him to be credible, concluding that the copied documents could be faked. He then went to the British embassy and the Secret Intelligence Service (MI6) officer there saw his potential and following consultations with London accepted him as an agent. Following were operations to retrieve the 25,000 pages of files hidden in his house, covering operations from as far back as the 1930s. He and his family were then smuggled out of Russia to Britain.
Richard Tomlinson, the MI6 officer imprisoned in 1997 for attempting to publish a book about his career, was one of those involved in retrieving the documents from empty milk cartons hidden under the floor of the dacha.
Spies exposed as a result of the defection include: