Hagerthy was 18 and playing the gentleman caller in a Glendale College production of The Glass Menagerie in 1950 when he was invited to Warner Bros. for an interview. Hagerthy's first screen role was as 19-year-old Dick Cvetic in the 1951 Warner Brothers film, I Was a Communist for the FBI. Frank Lovejoy (1912–1962) played the role of Matt Cvetic, a 39-year-old undercover Federal Bureau of Investigation agent within the Communist Party United States of America. Philip Carey (1925–2009) and Paul Picerni (1922–2011) were cast in the roles of Mason and Joe Cvetic, respectively. I was a Communist in the FBI subsequently inspired the syndicated television series I Led Three Lives. Hagerthy also appeared in small roles in two other 1951 Warner Bros. films, as Minto in Force of Arms and as corporal Rick Williams, who become the romantic interest of a Hollywood starlet, played by Janice Rule, in Starlift.