Cathy Williams traveled with the 8th Indiana, accompanying the soldiers on their marches through Arkansas, Louisiana, and Georgia. She was present at the Battle of Pea Ridge and the Red River Campaign. At one time she was transferred to Little Rock, where she would have seen uniformed African-American men serving as soldiers, which may have inspired her own interest in military service. Later, Williams was transferred to Washington, D.C., where she served with General Philip Sheridan’s command. When the war ended, Williams was working at Jefferson Barracks.

Despite the prohibition against women serving in the military, Williams enlisted in the United States Regular Army on November 15,1866 at St. Louis, Missouri for a three year engagement, passing herself off as a man. She was assigned to the 38th infantry. Only two others are known to have been privy to the deception, her cousin and a friend, both of whom were fellow soldiers in her regiment.