Prior to the United States entering the First World War, Fish was captain of Company K, 15th New York Infantry.
When the 15th was mobilized for Federal service, Fish accepted an offer from Col.
William Hayward to retain his position in the 369th Infantry (as the 15th New York was re-designated following mobilization).
The 369th was a unit of African American enlisted men with white officers (and a few African American officers at the start of the war) which came to be known as the "Harlem Hellfighters."
The 369th Infantry was assigned to the 93d Division.

The summer after President Wilson's declaration of war against Germany (in April 1917), Fish and about two thousand soldiers began training at Camp Whitman (in New York).
In October 1917, the unit was ordered to Camp Wadsworth (in South Carolina) for further training.
In November 1917, the regiment boarded the USS Pocahontas, destined for France, although shortly thereafter the ship returned to shore due to engine problems.
After another aborted departure, the ship left on December 13, 1917.
Despite colliding with another ship and not having a destroyer escort to protect against German submarines, the regiment reached France. (
Fish complained to Assistant Secretary of the Navy Franklin D. Roosevelt about the lack of an escort.)

Fish and his unit landed in Brest, France on December 26; the 369th Infantry was placed under the control of the French army by U.S. General John J. Pershing.
Altogether, the 369th Infantry spent 191 days on the front lines, which was the longest of any American regiment.
It was also the first Allied regiment to reach the Rhine River.
Fish received the Silver Star and the French War Cross 1914–1918.
In addition, Fish and his sister Janet, who had been a nurse near the front lines, were both later inducted into the French Legion of Honor for their wartime service.

Fish was promoted to major on March 13, 1919 and returned to the United States on April 25 of the same year.
He was discharged from the Army on May 14, 1919.
 He continued as a member of the Officers' Reserve Corps until the 1940s, and attained the rank of colonel.
Prior to the United States entering the First World War, Fish was captain of Company K, 15th New York Infantry. When the 15th was mobilized for Federal service, Fish accepted an offer from Col. William Hayward to retain his position in the 369th Infantry (as the 15th New York was re-designated following mobilization). The 369th was a unit of African American enlisted men with white officers (and a few African American officers at the start of the war) which came to be known as the "Harlem Hellfighters." The 369th Infantry was assigned to the 93d Division.
The summer after President Wilson's declaration of war against Germany (in April 1917), Fish and about two thousand soldiers began training at Camp Whitman (in New York). In October 1917, the unit was ordered to Camp Wadsworth (in South Carolina) for further training. In November 1917, the regiment boarded the USS Pocahontas, destined for France, although shortly thereafter the ship returned to shore due to engine problems. After another aborted departure, the ship left on December 13, 1917. Despite colliding with another ship and not having a destroyer escort to protect against German submarines, the regiment reached France. (Fish complained to Assistant Secretary of the Navy Franklin D. Roosevelt about the lack of an escort.)
Fish and his unit landed in Brest, France on December 26; the 369th Infantry was placed under the control of the French army by U.S. General John J. Pershing. Altogether, the 369th Infantry spent 191 days on the front lines, which was the longest of any American regiment. It was also the first Allied regiment to reach the Rhine River. Fish received the Silver Star and the French War Cross 1914–1918. In addition, Fish and his sister Janet, who had been a nurse near the front lines, were both later inducted into the French Legion of Honor for their wartime service.
Fish was promoted to major on March 13, 1919 and returned to the United States on April 25 of the same year. He was discharged from the Army on May 14, 1919.  He continued as a member of the Officers' Reserve Corps until the 1940s, and attained the rank of colonel.
Hamilton Fish III