Allen Marsh is an American actor. He was born at MCAS Cherry Point, North Carolina to Mary Jo (Allen), a teacher, and Joseph Marsh, a pastor. The family moved frequently, and by the time he was 17 he had attended eight different schools in six states. He was president of his high school's drama club and studied theatre in college before serving in the Air Force, working first at the 548th Reconnaissance Technical Group at Hickam Air Force Base, Hawaii, and later at the Joint Intelligence Center Pacific at Pearl Harbor. Afterward he worked at various jobs including graphic designer, closed captioning editor, flight attendant, and radio announcer before an decision to audition for a play led to a role in a stage production of You Can't Take It with You (1938).

He is best known for playing screenwriter Alvin Sargent on The Offer (2022) opposite Josh Zuckerman and Colin Hanks. He portrayed C.S. Lewis on Tolkien's the Lord of the Rings: A Catholic Worldview (2011) and televangelist Jim Bakker on The Con (2020). He played The Rocky Horror Picture Show (1975) producer Lou Adler on Winning Time: The Rise of the Lakers Dynasty (2022), makes frequent appearances as pianist Les Thomas at the Genoa City Athletic Club's Neil Winters Jazz Lounge on The Young and the Restless (1973), and plays Dr. Neal Elattrache on Ryan Murphy's American Sports Story (2024).