George Herman Mahon was born on September 22, 1900, near Haynesville, Louisiana to John Kirkpatrick and Lola Willis (Brown) Mahon. In 1908 the family moved to Loraine, Mitchell County, Texas. Mahon graduated from Loraine High School in 1918. On December 21, 1923 he married Helen Stephenson. The couple had one daughter. Mahon received his BA in 1924 from Simmons College (later known as Hardin-Simmons). He graduated from the University of Texas Law School in 1925. After additional postgraduate work at the University of Minnesota, he became a practicing lawyer in Colorado City, Texas in 1925.

The following year, Mahon was elected as the Mitchell County Attorney and in 1927 was appointed district attorney for the Thirty-second Judicial District by Texas Governor Dan Moody. The Democrat served two terms as district attorney from 1928-1932, after which he won the congressional seat for the Nineteenth Congressional District in 1934. He remained in office forty-four years. In 1978 the West Texan retired as the longest sitting member of Congress.

Mahon served on a number of House committees, including: insular affairs, elections, civil service, and census. In 1939 he gained a seat on the powerful House Appropriations Committee. Beginning in 1940 the leader secured membership on the War Department Appropriations Subcommittee (later known as the Defense Appropriations Subcommittee). He served as chairman of the subcommittee from 1949 to 1952 and 1955 to 1978. He was considered an expert in defense matters and become one of a handful of congressmen who had early knowledge about the Manhattan Project, which developed the atomic bomb during World War II. Mahon, an advocate of limited government spending, became chairman of the Joint Senate-House Committee on Reduction of Federal Expenditures. He also served on the Joint Senate-House Study Committee on Budget Control, as well as on the President's Commission on Budget Concepts. Ultimately, in 1964, the Texan became the chairman of the House Committee on Appropriations, holding that influential post until his retirement in 1978.

Mahon became a member of the Smithsonian Institution board of regents and won numerous awards for his public service, including: the George Washington Award of the Good Government Society; the Man of the Year Award of the Reserve Officers Association, the Distinguished Service Award of the American Political Science Association, and the Veterans of Foreign Wars Distinguished Public Service Award for Outstanding Service to the Nation. He was granted honorary degrees by Waynesburg College, Pepperdine College, Wayland Baptist College, Hardin-Simmons University, and Texas Technological College. In Lubbock, Texas, the George and Helen Mahon Library, the Texas Tech University Mahon Professorship in Law, and the George Mahon Elementary School were all named in his honor.