At the outbreak of the American Civil War Le Moyne enlisted as a first lieutenant with Company B of the 28th Massachusetts Volunteers under the command of fellow actor Lawrence Barrett. At some point Barrett resigned and Le Moyne assumed command only to witness over half his men killed or wounded in a string of Northern defeats in South Carolina and Virginia. In September 1862, Le Moyne himself was severely wounded during the Battle of South Mountain and was unable to return to military service. He was later granted by congress a retroactive promotion to the rank of captain dating back to the point he assumed command of company B.