Following his graduation, Duncan returned to Knoxville, where he had accepted a position as assistant attorney general of Knox County.

[3][7] In late 1952, Duncan became embroiled in a local controversy when, as commander of the American Legion's East Tennessee Division, he drafted a resolution condemning University of Tennessee's film society for a planned showing of several films starring Charlie Chaplin, who had been accused of being a communist sympathizer.

[8][9] Another contentious issue erupted in the Summer of 1960, when several black students from Knoxville College initiated a series of sit-ins to protest segregation at downtown-area lunch counters.

[10] With the backing of the Chamber of Commerce, Duncan formed a Good Will Committee, which encouraged downtown businesses to integrate their lunch counters.

[10] Duncan's early intervention in the crisis is often cited as one of the reasons Knoxville avoided the widespread integration-related violence that plagued other Southern cities during this period.

[12] In 1964, roughly 10 months into his second term as mayor, Duncan won a hard-fought Republican primary election in the Knoxville-based Second Congressional District.

The district's seven-term incumbent, Howard Baker Sr., had died that January, and his wife Irene held the seat for the rest of his term as a caretaker.

Duncan was heavily favored due to his popularity as mayor of Knoxville and the heavy Republican tilt of the district.

The contest was closer than expected in part because the 2nd was nearly swept up in Lyndon Johnson's national landslide in that year's presidential election; Barry Goldwater just barely carried it.