Working on the battle front from August 1936 to the middle of 1938, Palmer rode with ambulance drivers and kept records of the wounded, sending injury and death reports to the central war office. During the Battle of Brunete, fighting was intense and the field hospital was close to the action, which led Palmer to feel depressed and unstable. She asked for leave, which was finally granted in August 1937. Though her parents urged her to remain in England, Palmer returned to Spain the following month, where she joined the 35th Hospital Division in Aragon. By December, the unit was in Teruel, in sub-zero temperatures, with food and supply shortages and intense fighting, retreat and recapture. Eventually the medical staff were evacuated and returned to a medical hospital in Barcelona. When Franco's troops divided Spain in half, in May 1938, Palmer returned to London. She worked for the National Joint Committee for Spanish Relief through 1939, distributing leaflets, carrying out publicity events, and attending rallies. She wrote a novel, "Last Mile to Huesca" about her time in Spain, though it remained unpublished.