Born in Ogdensburg, New York, Chapin enlisted in the Army from Pamelia, New York in February 1864 for a 1-year term. He was serving in the 142nd New York Volunteer Infantry when his regiment attacked at the head of the Union column against Confederate fortifications at the Second Battle of Fort Fisher, North Carolina on January 15, 1865. Although just 17 years old, Chapin voluntarily advanced at the head of his regiment, and despite heavy hostile fire, he gallantly cut down the enemy palisading, allowing his comrades to breach the fortifications and achieve victory. He was transferred to the 169th New York Infantry in June 1865, and mustered out with his regiment the following month.