In 1945, Chňoupek became a member of the Communist Party of Czechoslovakia. Since 1948 he worked as journalist. After defeat of the Prague Spring, he became a member of the Central Committee of the Communist Party and obtained important government positions: 1969–70 directory of the state radio, 1970–71 ambassador in the Soviet Union, 1971–1988 minister of foreign affairs. Chňoupek always supported politics of the Soviet Union, including the perestroika campaign in late 1980s. After the fall of socialism in Czechoslovakia in 1989, he was investigated by police for misuse of powers and spent half a year in prison.