Shevchenko joined the Soviet diplomatic service, the Ministry of Foreign Affairs, as a young man and rose through its ranks to become an advisor to Foreign Minuster Andrei Gromyko. In 1973, he was appointed Under-Secretary-General of the United Nations (USG) During his assignment at the UN headquarters, in New York City, Shevchenko began to pass Soviet secrets to the CIA because he could not objectively fulfill his mission of impartiality to the United Nations. In 1978, he cut his ties to the Soviet Union and defected to the United States, where he lived for the rest of his life.