During World War I, he worked as a medical minder with the Royal Army Medical Corps. After working with the French oral and maxillofacial surgeon Hippolyte Morestin on skin grafts, he persuaded the army's chief surgeon, Arbuthnot-Lane, to establish a facial injury ward at the Cambridge Military Hospital, Aldershot, later upgraded to a new hospital for facial repairs at Sidcup in 1917. There Gillies and his colleagues developed many techniques of plastic surgery; more than 11,000 operations were performed on more than 5,000 men (mostly soldiers with facial injuries, usually from gunshot wounds). After the war, Gillies developed a private practice with Rainsford Mowlem, including many famous patients, and travelled extensively to promote his advanced techniques worldwide.
During World War I, he worked as a medical minder with the Royal Army Medical Corps After working with the French oral and maxillofacial surgeon Hippolyte Morestin on skin grafts, he persuaded the army's chief surgeon, Arbuthnot-Lane, to establish a facial injury ward at the Cambridge Military Hospital, Aldershot, later upgraded to a new hospital for facial repairs at Sidcup in 1917. There Gillies and his colleagues developed many techniques of plastic surgery; more than 11,000 operations were performed on more than 5,000 men (mostly soldiers with facial injuries, usually from gunshot wounds). After the war, Gillies developed a private practice with Rainsford Mowlem, including many famous patients, and travelled extensively to promote his advanced techniques worldwide.