The 12th Maine Regiment was mustered in for three years' service on November 20, 1861, and were mustered out on December 7, 1864. Companies A, B, C, and D became part of the Battalion 12th Regiment Infantry. The Battalion was mustered out April 18, 1866 at Savannah, Georgia. Of the men enrolled in the regiment, 52 were killed or died of wounds and 239 died of disease.
They served in the following engagements:
- Pass Manchac, LA - June 17, 1862
- Irish Bend, LA - April 14, 1863
- Port Hudson, LA - May 27-July 7, 1863
- Donaldsonville, LA - July 13, 1863
- Winchester, VA - September 19, 1864
- Cedar Creek, VA - October 19, 1864