He was appointed U.S Ambassador to the Soviet Union in 1943, leaving that post in 1946 to become, briefly, Ambassador to the United Kingdom, and then Secretary of Commerce. Harriman served as national security adviser during the Korean War.
Harriman made a successful bid for the governorship of New York in 1954, serving from 1955-59, and lost a Democratic nomination bid for the Presidency in 1956.
He served as a roving ambassador for the Kennedy White House, and as Undersecretary of State and as an ambassador-at-large under Lyndon Johnson. Harriman was the chief U.S. negotiator at the Paris peace talks on Vietnam.