Janssen appeared in many television series before he landed programs of his own. In 1956, he and Peter Breck appeared in John Bromfield's syndicated series "Sheriff of Cochise" in the episode "The Turkey Farmers". Later, he guest-starred on NBC's medical drama "The Eleventh Hour" in the role of Hal Kincaid in the 1962 episode "Make Me a Place", with series co-stars Wendell Corey and Jack Ging. He joined friend Martin Milner in a 1962 episode of "Route 66" as the character Kamo in the episode "One Tiger to a Hill."