Washburn was a leader in both civilian and military medical administration. He was the Director of Massachusetts General Hospital from 1908 to 1934, overseeing the hospital during one of its largest periods of expansion. The hospital built the orthopedic ward and the Walcott House and created a new system of hospitals-within-a-hospital to appeal to patients of different social classes. The Baker House cared for patients of moderate means who could pay for treatment, unlike in the main hospital wards, which saw a mix of patients who paid and who were provided for by philanthropic contributions. The Phillips House provided high-end accommodations for wealthy patients. During World War I, Washburn organized the Mass General-run U.S. Army Base Hospital No. 6. The hospital was staffed by Mass General doctors and nurses and served more than 4,000 patients. Washburn was put in charge of American hospitals in England, and he was decorated by both the American and British governments. After retiring he wrote The Massachusetts General Hospital: Its Development, 1900-1935.