Sisson was commissioned as a lieutenant and paymaster for the 1st Rhode Island Detached Militia (aka.
1st Rhode Island Volunteer Infantry Regiment) in May 1861 and served until the regiment was mustered out in August 1861. He participated in the
Battle of Bull Run on July 21, 1861 under General
Ambrose Burnside. In April 1862 he was commissioned as major of the
3rd Rhode Island Heavy Artillery and commanded three companies (B, F and K) of that regiment at the
Battle of Secessionville, South Carolina. On November 5, 1862 he was promoted to colonel and given command of the 5th Rhode Island Infantry which became the
5th Rhode Island Heavy Artillery on May 27, 1863. Colonel Sisson was mustered out of service, along with his regiment, on June 26, 1865. ==Post-war life==