Among his film roles Marshall is perhaps best known as the unflappable and analytical Juror 4 in Sidney Lumet's courtroom drama 12 Angry Men (1957).

Although most familiar for his later television and movie roles, which gained wide audiences, Marshall also had a distinguished Broadway career.

In 1948, having already performed in the original New York productions of The Skin of Our Teeth and The Iceman Cometh, Marshall joined Marlon Brando, Montgomery Clift, Julie Harris, Kim Stanley, and 45 others to make up the first group of actors granted membership in the newly formed Actors Studio.

[9] Marshall achieved perhaps his highest profile as top-billed star of the CBS-TV legal drama The Defenders (1961-5).

He later played Dr. David Craig in the television series The Bold Ones: The New Doctors (1969–73), and Nazi collaborator Henri Denault on the CBS prime-time drama Falcon Crest in 1982.

His final performance was a reprisal of his role as Lawrence Preston in two TV Movies based on The Defenders.

[15] During the 1968 United States presidential campaign, he filmed and narrated a political advertisement endorsing Democratic candidate Hubert Humphrey.