 During World War I, Carstairs served in France with the American Red Cross, driving ambulances.After the war, she served with the Royal Army Service Corps in France, re-burying the war-dead, and in Dublin with the Women's Legion Mechanical Transport Section, which acted as transport for British officers during the Irish War of Independence.Joe Carstairs married a childhood friend, the French aristocrat Count Jacques de Pret, on 7 January 1918 in Paris.