In 1957, Joslyn co-starred in eight episodes as literary agent George Howell in the short-lived CBS sitcom "The Eve Arden Show". He also guest-starred in the NBC western series "The Californians". In 1959, he was cast as Quag in the episode "Gold Sled" of the ABC/Warner Brothers western series "The Alaskans", starring Roger Moore. In the 1960–1961 season, Joslyn guest-starred on Pat O'Brien's ABC sitcom "Harrigan and Son". He played one-half of the title role on the 1962 TV sitcom "McKeever And The Colonel". That same year, he guest-starred in ABC's crime drama series "" with Stephen McNally and Robert Harland. He was featured as well in some episodes of ABC's "The Addams Family" playing the role of beleaguered Sam L. Hilliard, whose efforts as a school administrator and politician were continually undone by the Addams family.