Adams served in the Union Army during the American Civil War. He was commissioned as a first lieutenant in the 1st Massachusetts Volunteer Cavalry on December 28, 1861. He was promoted to captain on December 1, 1862. He fought with distinction during the Gettysburg Campaign, where his company was heavily engaged at the Battle of Aldie. When the regiment's 3-years enlistment ended it was reduced to a battalion; and Adams was mustered out of service on September 1, 1864.