Born in Buffalo, Missouri on June 6, 1914 to Philip Allen Bennett and Mary Bertha (Tinsley) Bennett, he received his A.B. from Southwest Missouri State University in 1935 and earned a J.D. from the School of Law at Washington University in St. Louis in 1938, after which he entered private practice. In 1941 he married June Young; they had two children, William and Ann, and four grandchildren, Jiffy, James, John, and Anna.
While practicing law, he was elected to the Greene County, Missouri Republican Central Committee, and managed his father's two Congressional election campaigns. When his father died in office in early 1943, Marion was chosen in a special election to complete his father's term in the 78th Congress. He was 28 years old. Winning reelection twice, he served in the 79th and 80th Congresses.Marion Bennett was part of an official ten member Congressional delegation invited by General Dwight D. Eisenhower to follow the United States Third Army into Buchenwald concentration camp near Weimar, Germany in 1945. The delegation's members toured the United States speaking and reporting on what they had seen of the horrific conditions in the camp. Bennett is quoted in the New York Times on May 6, 1945, "I left Buchenwald convinced that every German must be killed."Losing his reelection bid in 1948, Bennett was appointed a trial commissioner to the federal Court of Claims, starting the day after his term ended, January 4, 1949. In 1950 he became an Air Force Reserve Colonel, a position he held until 1974. He also rose to become Chief Trial Commissioner at the Court of Claims in 1964, and was nominated in 1972 to be an Associate Judge. By enactment of the Federal Courts Improvement Act of 1982 he became a Circuit Judge of the U.S. Court of Appeals for the Federal Circuit. He became a Senior Circuit Judge on March 1, 1986, and continued to sit until 1997. He died in Alexandria, Virginia, on September 6, 2000, and is buried in Springfield, Missouri. Bibliography American immigration policies : a history by Marion T. Bennett. Washington : Public Affairs Press, c1963. Private claims act and congressional references . Washington : U.S. Govt. Print. Off., 1968. The United States Court of Claims : a history, pt. 1. The judges, 1855-1976, by Marion T. Bennett / pt. 2. Origin, development, jurisdiction, 1855-1978, W. Cowen, P. Nichols, M.T. Bennett. Washington, D.C. : Committee on the Bicentennial of Independence and the Constitution of the Judicial Conference of the United States, 1976 i.e. 1977-1978. 2 vol.