John J. Duncan Sr., congressman and mayor of Knoxville, was born on a farm in Scott County, the sixth of ten children of F. B. and Cassie Duncan. Duncan attended the University of Tennessee, Knoxville, and in 1942, while a student there, married Lois Swisher. During World War II he served as a criminal investigator for the U.S. Army. After the war Duncan attended Cumberland University and the University of Tennessee, where he received his LL.B. and J.D. degrees in 1948. From 1948 to 1956 he served as state assistant attorney general.
Duncan's political career began with his work in Howard Baker Sr.'s victorious 1954 campaign for Congress from the Second District. In 1959 Duncan was elected mayor of Knoxville. As a southern mayor during the civil rights era, Duncan worked with sit-in demonstrators to avoid violence and accomplish the peaceful integration of Knoxville businesses. In 1964 he won election to Congress to the seat previously held by Baker. Duncan gained a reputation as a low-profile congressman who focused on the problems of his constituents. Politically conservative, Duncan was a Vietnam War hawk who advocated get-tough policies against antiwar demonstrators. Still, he maintained a working relationship with many who did not support his conservative positions, including the NAACP and environmental groups concerned with protection for the Great Smoky Mountains National Park.
Duncan represented the Second Congressional District from 1964 until his death from prostate cancer on June 21, 1988. His son, John J. Duncan Jr., was elected to replace him in Congress and continues to serve in 2001.