On November 18, 1861, at the age of 20, Thomas A. Anderson enrolled for military service at Wheeling, West Virginia. He then officially mustered in for duty with Company I of the 1st West Virginia Cavalry. Assigned to guard lines of the Baltimore and Ohio Railroad in what is now West Virginia during the opening months of 1862, Anderson and his fellow 1st Virginia Cavalrymen were directed to combat assignments beginning that spring. After fighting at Winchester, Virginia (March 23), they engaged the enemy in operations at Monterey (April 12), Buffalo Gap (May 3), McDowell (May 7), Strasburg (June 1), Cross Keys (June 8), Port Republic (June 9), and White Plains (June 10). Marched east of the Appalachian Mountains, they then re-engaged with the enemy in Culpeper, Fauquier and Fairfax counties, including battles at Cedar Mountain (August 9) and Orange Court House (August 13). Next engaged in Union General John Pope's Northern Virginia Campaign, they participated in operations along the Rapidan River (August 18), Freeman's Ford/Hazel River (August 22), Kelly's Ford (August 22), Sulphur Springs (August 23), Waterloo Bridge (August 23–August 25), and Gainesville (August 28), as well as in the Second Battle of Bull Run (August 28–30). Also engaged in operations at Lewis Ford (August 30), they then fought in the Battle of Chantilly (September 1).