Abraham Lincoln's first call for volunteers in April, 1861 required Maine to raise one regiment of infantry for three months of Federal service. This was done by reorganizing ten existing companies of the state militia, completed at Portland, Maine on 28 April 1861 and mustered into service on 3 May 1861, a total of 779 soldiers. The regimental commander was Colonel Nathaniel Jackson.

The First Maine was transferred to Washington, D.C. on 1 June 1861, where it remained until 1 August 1861, encamped on Meridian Hill. It spent its entire service in the Washington defenses and saw no combat. They were mustered out on 5 August 1861.