The 3rd Maryland Infantry, Potomac Home Brigade was organized at Cumberland, Hagerstown, and Baltimore, Maryland, beginning October 31, 1861, and mustered in on May 20, 1862, for three years under the command of Colonel Henry C. Rizer. Companies I and K were organized at Ellicott's Mills and Monrovia, Maryland, in April and May 1864. Although the 3rd served throughout the war in the Virginia Theater, they did not get involved in most of the major battles. Their major battles were at Harper's Ferry and Monocacy. The regiment mustered out of the service at Baltimore on May 29, 1865.