In 1928 Cooper passed the Kentucky bar examination and soon became active in state politics. Volunteering for military service in 1942, he was commissioned an officer the following year, serving in France, Luxembourg, and Germany and helping to reorganize Bavaria's judicial system in 1945 before his discharge as a captain.
Appointed to fill an unexpired Kentucky U.S. senatorial term from 1946 to 1948, Cooper, although a Republican, often voted with the Democrats. Seeking to maintain bipartisan support for his foreign policies, in 1949, 1950, and 1951 President Harry S. Truman named Cooper a delegate to the United Nations General Assembly. Cooper, a staunch anti-communist who firmly supported military cooperation with Western Europe, also advised Secretary of State Dean Acheson at a 1950 London-Brussels North Atlantic Treaty Organization (NATO) conference. From 1952 to 1955 Cooper again succeeded to a vacant unexpired Kentucky senatorial seat. Appointed ambassador to India and Nepal by President Dwight D. Eisenhower from 1955 to 1956, Cooper showed tact and sensitivity in dealing with the often prickly Prime Minister Jawaharlal Nehru, which helped to allay the often difficult relationship between the two countries.
In 1956 Cooper won election to the Senate from Kentucky in his own right, holding his seat until 1973 and winning a reputation for political independence. One of the first senators to condemn Senator Joseph McCarthy's tactics of harassing alleged communist sympathizers, even in 1954 Cooper opposed U.S. military intervention in Vietnam.
Seeking to maintain national unity and under pressure from the Democratic Senate leadership, in 1964 Cooper voted for the Tonkin Gulf Resolution giving President Lyndon B. Johnson wide discretionary powers to handle the deteriorating situation in Vietnam, a decision that Cooper subsequently regretted. He soon became a strong critic of the war, opposing escalation of U.S. bombing of the Democratic Republic of Vietnam (DRV, North Vietnam) and the commitment of ground forces and questioning presidential powers for continued expansion of the American war effort without congressional authorization. In 1970, in response to the secret bombing raids and eventual U.S. invasion of Cambodia ordered by President Richard Nixon and National Security Advisor Henry A. Kissinger, Cooper, together with Senator Frank Church, coauthored the Cooper-Church Amendment, which barred funding for U.S. military forces in Cambodia, although this only passed six months after the troops were withdrawn.
Cooper resigned his Senate seat in 1973 and resumed his law practice in Washington, D.C. From 1974 to 1976 he was the first U.S. ambassador to East Germany. Cooper died in Washington, D.C., on 21 February 1991.
Bamberg, James. British Petroleum and Global Oil, 1950-75: The Challenge to Nationalism. Cambridge: Cambridge University Press, 2000.; Schulman, Robert. John Sherman Cooper: The Global Kentuckian. Lexington: University Press of Kentucky, 1976.; Smoot, Richard C. "John Sherman Cooper: The Paradox of a Liberal Republican in Kentucky Politics." Unpublished PhD diss., University of Kentucky, 1988.