He was known as a great athlete at Harvard, despite serious eye problems that forced him to leave school in his senior year and have his lessons read to him in order to graduate.
In 1861, Massachusetts Governor John Albion Andrew appointed Winsor head of the State Hospital at Rainsford Island.
A year later he left his post to enlist as a surgeon in the 49th Massachusetts regiment, serving from 1862 to 1863 through the disastrous Louisiana Campaign under the command of Colonel (later General) William Francis Bartlett.
During a disastrous frontal assault on Port Hudson, Louisiana on May 27, 1863, Bartlett was shot twice—once in his remaining unamputated leg and once in his wrist, shattering it.
He would write a gripping account of that night years later for Atlantic Monthly: Although Winsor returned to private practice in Winchester, Massachusetts after his service, he continued to contribute to the State Board of Health, writing important state health reports such as "The Hygiene of School Houses" (1874), and "Water Supply, Drainage, and Sewerage from the Sanitary Point of View" (1876).