In 1931, Austin was elected to the U.S. Senate in a special election, defeating Frank C. Partridge, who had been appointed to fill the vacancy after the death of Frank L. Greene. From 1939 to 1942, Austin was the Senate's Assistant Minority Leader (Minority Whip). In 1946 he resigned to accept president Harry S. Truman's appointment as U.S. Ambassador to the United Nations. He took office in January, 1947, and was the first official holder of this post. Austin served until January 1953, when the Eisenhower administration took office. He resided in Burlington during his retirement, and died there in 1962. Austin was buried at Lakeview Cemetery in Burlington.