Green’s Dictionary of Slang

(US prison) a prison guard who works in the prison yard, controlling the prisoners.

1929		C.G. Givens ‘Chatter of Guns’ in Sat. Eve. Post 13 Apr.; list extracted in AS VI:2 (1930) 133: herder, n. Prison guard.
1936		San Quentin Bulletin in L.A. Times 6 May 7: HERDER, guard in prison.
1945		J. Fishman Names or Numbers 63: Can’t a guy make his time without some ‘herder’ always picking on him?
1949		Monteleone Criminal Sl. (rev. edn).
1992		Bentley & Corbett Prison Sl. 96: Herder A prison guard assigned to yard duties where he mingles amongst the prisoners on the prison grounds.