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 regiment consisted of Company A - Portland Light Infantry, Company B - Portland Mechanic Blues, Company C - Portland Light Guards, Company D - Portland Rifle Corps, Company E - Portland Rifle Guards, Company F - Lewiston Light Infantry, Company G - Norway Light Infantry, Company H - Auburn Artillery, Company I - Portland Rifle Guards, and Company K - Lewiston Zouaves

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.

While the 1st Maine was a 3-month regiment, many had enlisted for longer or were willing to reenlist and though they wanted to retain their old designation, the men were incorporated into the 10th Maine and later the 29th Maine Infantry.