The 19th Maine Regiment was mustered in for three years' service on August 25, 1862, and were mustered out on May 31, 1865. In November 1864 the 5th Company of Unassigned Maine Infantry was incorporated, and in conjunction with the 15th Regiment Massachusetts Infantry, the 1st Regiment Minnesota Infantry and the 82nd Regiment New York Infantry, it formed Harrow's 1st Brigade, Gibbons's 2nd Division, 2nd Corps. This Brigade had the greatest percentage of casualties of any brigade at Gettysburg, where its loss was 61 percent of those engaged. Of the 1441 men enrolled in the regiment, 192 were killed or died of wounds, 501 were wounded, 184 died of disease, and 47 were in Confederate prisons.

They served in the following engagements:

• Fredericksburg, VA - December 13, 1862
• Chancellorsville, VA - May 1 - 4, 1863
• Gettysburg, PA - July 1 - 3, 1863
• Bristoe Station, VA - October 14, 1863
• Mine Run, VA - November 26 - December 1, 1863
• Wilderness, VA - May 5 - 7, 1864
• Spotsylvania, VA - May 7 - 20, 1864
• Po River, VA - May 9 - 12, 1864
• North Anna, VA - May 23 - 27, 1864
• Totopotomy, VA - May 26, - 30, 1864
• Cold Harbor, VA - May 31 - June 12, 1864v
• Petersburg, VA Seige - June 1864
• Deep Bottom, VA - August 14, 1864
• Reams Station, VA - August 25, 1864
• Boydton Road, VA - October 23, 1864
• Hatchers Run, VA - December 8 - 9, 1864
• High Bridge, VA - April 6, 1865
• Farmville, VA - April 7, 1865
• Appomattox, VA - April 9, 1865