Henderson was a keen supporter of the Republican cause and anti-fascist forces during the Spanish Civil War. He joined the Spanish Medical Aid Committee (SMAC) when it was first established in 1936, and later travelled to Spain to visit numerous front-line hospitals. During the war, Henderson donated his Rolls-Royce car and had it converted into a makeshift ambulance to serve republicans and anti-fascists. This ambulance saw frequent use and was in action during the Battle of Teruel where it was used to evacuate people with serious abdominal wounds. Henderson's Rolls-Royce ambulance, though heavily damaged and covered in bullet holes, survived the war and was shipped back to Britain in September 1938. The ambulance was then displayed at a conference for officials of the UK Trades Union Congress (TUC), where it was used to raise monetary funds for Spanish medical programmes.