The unit was created in March 1937, from troops from the 20th Mixed Brigade, as well as the "Extremadura" battalions No.

It was under the command of Juan García Pina, with the anarcho-syndicalist Germán Clemente de la Cruz as political commissar.

In July 1938, at the beginning of the nationalist offensive on the Extremadura front, the 91st Mixed Brigade was located opposite Peraleda del Zaucejo.

After the start of the fighting, the head of the unit, Ignacio López Montilla, was taken prisoner by the nationalists, having to be replaced by Copérnico Ballester Francés; but Ballester was wounded during the fighting, so the command of the brigade passed to Olegario Pachón Núñez.

Around July 20, the 91st MB defended the southern front of the Mérida pocket, in Guareña.