Keillor's earliest radio experience came as a student at the University of Minnesota, where he graduated as a journalism major in 1966. Beginning in 1969, Keillor wrote for The New Yorker magazine. While researching an article on the Grand Ole Opry in 1974, Keillor conceived a live radio show featuring traditional music. While the first broadcasts of A Prairie Home Companion were heard on Minnesota Public Radio, the show quickly became a national phenomenon. During the first 13 years the show was aired, it received the George Foster Peabody award, the Edward R. Murrow award, and a medal from the American Academy of Arts & Letters. Although Keillor's deadpan delivery remained the show's centerpiece, A Prairie Home Companion provided regular airplay for a lengthy list of performers including Greg Brown, Jean Redpath, Bill Staines, Beausoleil, Robin & Linda Williams, Butch Thompson, Prudence Johnson and Michael Cooney. The show was broadcast on the Disney cable-TV station for everal years and received two Ace awards for television broadcast excellence.

In 1987, Keillor announced that he was ending A Prairie Home Companion. Moving to New York, he launched a similar show, The American Radio Company, two years later. Although the new show lasted four seasons, Keillor's fans wished for a return to the original program. In 1993, Keillor relented. Returning to the World Theater in St. Paul, Minnesota, he resurrected A Prairie Home Companion.

In addition to his continued involvement as host of A Prairie Home Companion, Keillor hosts a poetry radio show, The Writer's Almanac, broadcast daily and is a frequent contributor to The New York Times and The Atlantic magazine.

A deep-voiced vocalist, Keillor has sung many songs on A Prairie Home Companion. Together with the Hopeful Gospel Quartet, he recorded an album of spirituals and hymns in 1992.