The unit was created in May 1937, originally as a reserve force of the Eastern Army, although later it was integrated into the 32nd Division of the XI Army Corps. It was under the command of Humberto Gil Cabrera, with Manuel Tió Vila as chief of staff and with Luis Deltell Bernal as commissar. Once the brigade finished its training phase, the command passed to Rodolfo Bosch Pearson, but he ended up being replaced by Eusebio Sanz after Bosch was prosecuted for the execution of some militiamen from the POUM. Subsequently, the 140th Mixed Brigade was assigned to the 33rd Division in order to take part in the Battle of Brunete, although in the end the brigade remained located in Tarragona without taking part in the Brunete operations.