Still was the son of a Methodist minister and physician. At an early age, Still decided to follow in his father's footsteps as a physician. After studying medicine and serving an apprenticeship under his father, he entered the Civil War. He served as a hospital steward assigned to Company F of the Cass County Home Guard of the Missouri Cavalry (Union), but later stated in his autobiography that he served as a "de facto surgeon." Despite biographical accounts of him earning an MD there appears to be no record that he was ever conferred one.