The day after finishing as a house officer and becoming fully qualified to practise medicine, Henry enlisted in the Scottish Women's Hospitals for Foreign Service (SWH), working in the hospital set up in Royaumont Abbey, north-east of Paris. She arrived in July 1917 and was the youngest doctor on the staff. There, as an assistant surgeon, she had charge of the Blanche de Castille ward. She later served at the associated SWH hospital at Villers-Cotterêts, which treated French rather than British soldiers. After the war the French government awarded her the Croix de Guerre.
