Hopkins was mustered in as a private in Company I, First New Jersey Volunteer Infantry in 1861. In 1862, he was promoted to corporal. At the Battle of Gaines Mill, he carried a severely wounded comrade to safety while under fire. He was twice wounded while performing this brave act, and returned to the battle line. For this act, he was awarded the Confederate Medal of Honor in 1892. In 1863, he was captured, and spent part of his imprisonment at Andersonville. He was discharged in 1865.
