Despite the ban of women in the U.S. Army, Williams enlisted on November 15, 1866 in St. Louis. She served in the 38th U.S. Infantry, also known as the Buffalo Soldiers, for three years. Only her cousin and a close friend who served alongside her knew her true identity.
The truth about Cathay Williams was finally revealed when she was struck with smallpox and other illnesses from the harsh war conditions. On October 14, 1868, she was discharged from the Army once her post surgeon revealed that she was a woman to her commanding officer. After she returned from war, she worked as a cook in New Mexico. She married a man who later stole her savings and horses, whom she had arrested for his crimes. She developed diabetes and all of her toes were amputated. Though there had been other women who secretly served in the American Revolutionary War who obtained benefits, she was denied any disability or pension from the military.
Cathay Williams told her story to the St. Louis Daily Times in an article dated January 2, 1876. She died sometime in 1892. Unfortunately, her gravesite was never given a headstone and is its location is unknown.