[2] After this, he became interested in acting and joined several stock companies, where he gained experience and training as an actor in professional stage productions.

He starred with Victor McLaglen in John Ford's Mother Machree (1928), whose title became the catchphrase of Gordon's associate Chief O'Hara (played by Stafford Repp) on the Batman television series almost four decades later.

Hamilton was billed above newcomer Clark Gable in Laughing Sinners (1931), in which he played a cad who deserts Joan Crawford's brokenhearted character.

When television came along, Hamilton hosted Hollywood Screen Test (1948-1953), co-starred in the short-lived sitcom That Wonderful Guy with Jack Lemmon (1949–50), at the same time as Hollywood Screen Test, and did guest shots on numerous series of the 1950s and 1960s, such as seven episodes of Perry Mason: in 1958 he played murder victim Bertrand Allred in "The Case of the Lazy Lover" and Grove Dillingham in "The Case of the Drifting Dropout" in 1964.

He appeared on Maverick, Tales of Wells Fargo, The Real McCoys, Mister Ed, Bachelor Father, The Outer Limits, and The Cara Williams Show.

Producer Maury Dexter quickly signed Hamilton to replace Cromwell in the film, which co-starred Jimmie Rodgers and Chill Wills.

He also appeared as Police Commissioner James "Jim" Gordon in all 120 episodes of the Batman television series (1966–68) as well as the 1966 film of the same name.

Yvonne Craig, who played Commissioner Gordon's daughter Barbara, said Hamilton "came every day to the set letter perfect in dialogue and never missed a beat—a consummate professional.