The 7th Maine was organized at Augusta, Maine, and mustered into Federal service for a three-year enlistment on August 21, 1861. About 1,505 men served in the 7th Maine; of these, 152 were killed in action or died of their wounds; 403 men were wounded; another 212 died of various diseases while in the service, and 19 men perished in Confederate prisoner-of-war camps.
After the Battle of Antietam on September 17, their ranks thinned by the almost continuous campaigning, the 7th Maine was ordered home to recruit fresh troops in October. It remained in Portland, Maine, until January 21, 1863, when it joined the successful attack on Maryes Heights during the Second Battle of Fredericksburg on May 3 and the Battle of Gettysburg in July 1863. The regiment was mustered out at Charlestown, Virginia, on August 21, 1864. The veterans whose enlistments had not yet expired and the recent recruits were transferred to the 1st Maine Veteran Infantry.