Green’s Dictionary of Slang

1. (US) the court bench on which prisoners sit awaiting trial.

1904		Charlotte Observer (NC) 27 July 4: In the city police court a motley crowd of prisoners filled the mourners’ benches .
1912		D. Lowrie My Life in Prison 358: The boy seated himself on the ‘mourner’s bench’, and the Captain went into his office.
1935		A.J. Pollock Und. Speaks.

2. (US prison) a bench on which new inmates sit, e.g. for indoctrination talks.

1912		D. Lowrie My Life in Prison in Hamilton Men of the Und. 246: I sat on the ‘mourner’s bench’ and he stood before me, instructing me as to the rules.
1927		(con. 1900s) S. Lewis Elmer Gantry 29: Nerve of him, trying to lead me up to any mourners’ bench!

3. (US) the seats in a theatrical agent’s outer office where hopeful performers wait for jobs.

1921

N. Putnam West Broadway 16: I went out of the private office and through the crowd of [...] would-be picture stars who were waiting on the mourners’ bench.

4. (US black) a row of seats in a nightclub from which patrons can observe their fellows.

1941

D. Burley N.Y. Amsterdam Star-News 12 Apr. 13: Lawsy! The chicks sit on the mourner’s bench and move in on him [i.e. a handsome man] one by one.