During the First and Second World Wars many members of St John Ambulance from across the Cynon Valley volunteered to join the fighting abroad. Although this led to dwindling attendance at meetings the Divisions played an important role in the local war effort. Members volunteered for Air Raid Precaution, Home Guard (Civil Defence) and National Hospital Reserve duties. The Nursing Divisions and Nursing Cadets also assisted at the Blood Transfusion Centre and local hospitals.

Due to its industrial nature South Wales was a target for German bombers during the Second World War and there were fears that there may be large numbers of casualties. With the support from St John Officers, the Welsh Board of Health took over Aberdare Ambulance Hall in order to be prepared for any eventually.

Money was raised for the War Comforts Fund, which supported the families of St John Ambulance members who were serving in the war. Volunteers also trained others in the assembly and distribution of gas masks and lectured on their use, evacuation procedures and the care of the injured should there be a bomb attack in the area.