During its first months of service, the 21st Massachusetts was assigned garrison duty at the United States Naval Academy in Annapolis, Maryland. In January 1862, the regiment was attached to the Coast Division commanded by Major General Ambrose Burnside and embarked with the division for operations in North Carolina. Clark was placed in command of the regiment in February 1862 and led it in the Battle of New Bern on March 14, 1862. In that action, Clark garnered a reputation for bravery when the regiment charged a Confederate battery and he straddled an enemy cannon, urging his regiment forward. The gun was the first artillery piece captured by the Union Army during that engagement. It was presented by General Burnside to Amherst College in honor of Lieutenant Frazar Stearns, son of the president of Amherst College and adjutant of the 21st Massachusetts, who was killed in the battle. The cannon was mounted inside Morgan Hall at Amherst College.