Robert Emmet Hannegan was born on June 30, 1903, in St. Louis, Missouri, and received a law degree from St. Louis University in 1925. During the 1930s, Hannegan was active in Democratic politics on the local and state levels and helped get Harry Truman elected to the U.S. Senate in 1934. In 1942, he was appointed the collector of internal revenue for the eastern district of Missouri upon Truman's urging. An appointment as commissioner of internal revenue came his way the following year.

Hannegan was the chairman of the Democratic National Convention in 1944, and was appointed postmaster general when Truman became President in 1945. While in office, Hannegan streamlined and modernized the postal service. He resigned in 1947 due to health problems and died on October 6, 1949.