Most prominently, Conrad's deep, resonant voice was heard in the role of Marshal Matt Dillon on CBS Radio's gritty Western series "Gunsmoke" (1952–1961). The producers originally rejected him for the part because of his ubiquitous presence on so many radio dramas and the familiarity of his voice, but his impressive audition could not be dismissed, and he became the obvious choice for the role. Conrad voiced Dillon for the show's nine-year run, and he wrote the June 1953 episode "Sundown". When "Gunsmoke" was adapted for television in 1955, executives at CBS did not cast Conrad or his radio costars despite a campaign to get them to change their minds.