He became a professional soldier in the Weimar Republic in 1924 and started his career in intelligence in the Foreign Armies East unit that analysed the Soviet Union during WWII; before his 1955 appointment as director of MAD he worked for the CIA during the early 1950s.
[1] In 1946, he was delivered as prisoner of war from the British army to the Socialist Federal Republic of Yugoslavia, at the time a one-party state allied with Stalin.
Selmayr recounted his experiences in Tito's Yugoslavia in the book Die grosse Pause: Deutsche Soldaten in Titos Gewalt; according to the book he and his fellow soldiers were essentially hostages of the Yugoslav communist regime and he described the trials that took place there as political show trials based on political charges produced by the country's communist regime and unrecognised under international law; he and the other soldiers were released following the Tito–Stalin Split and the realignment of Yugoslav foreign policy that resulted in better relations with West Germany, after Chancellor Konrad Adenauer had negotiated with the Yugoslav government for the repatriation of the remaining German prisoners of war.
[3] In 1950 he made it back to West Germany, and in 1951 he was employed by the Gehlen Organization, a CIA-affiliated intelligence agency focused on the East European communist regimes, especially the Soviet Union.
In 1955 he was promoted to brigadier general in the West German Bundeswehr and appointed as the first director of the Military Counterintelligence Service, serving in the position for nine years until 1964.