In 1973, Duggan had a cameo appearance in the blaxploitation film Black Caesar. In 1975, he appeared as FBI Inspector Ryder in the NBC-TV movie Attack on Terror: The FBI vs. the Ku Klux Klan, and had roles in the 1976 TV miniseries Rich Man, Poor Man and Once an Eagle. In 1978, he appeared in the episode "And the Sea Shall Give Up Her Dead" of the NBC crime drama The Eddie Capra Mysteries. In 1980, he appeared as Sam Wiggins in the ABC television movie The Long Days of Summer, and later that same year guest-starred in an episode of the CBS series M*A*S*H* as Col. Alvin 'Howitzer Al' Houlihan, the legendary father of Margaret Houlihan, in the episode "Father's Day". One of Duggan's last parts was as Dwight D. Eisenhower in a TV biography called J. Edgar Hoover (1987), a role he had played earlier in the NBC mini-series, Backstairs at the White House (1979). He also played President Lyndon B. Johnson in a different biography of Hoover, The Private Files of J. Edgar Hoover (1977). He played "Judge Axel" in A Return to Salem's Lot (1987).