Born in Henderson, Texas, Leath attended the Rusk County public schools. He graduated from Henderson High School, 1949. He attended Kilgore Junior College. B.B.A., University of Texas, Austin, 1954. He served in United States Army from 1954 to 1956. Coached football and track, Henderson High School from 1957 to 1959. Business salesman, 1959. Banking, 1962. Officer and director in five Texas banks, and two manufacturing companies. He served as special assistant to United States Representative W.R. Poage from 1972 to 1974.
Leath was elected as a Democrat to the Ninety-sixth and to the five succeeding Congresses (January 3, 1979-January 3, 1991). He was not a candidate for renomination in 1990 to the One Hundred Second Congress.
In 1991 Leath founded the Washington, DC based Marvin Leath Associates, a consulting firm representing interests of military contractors. He ran his company until his death on December 8, 2000 in Arlington, Virginia.