Rosewater attended a commercial college. He then became an employee of a telegraph company. He worked in Oberlin, Ohio during 1859 during the celebrated abolitionist "cause célèbre", the Wellington rescue case. During that time Rosewater became associated with Simeon Bushnell and Charles Langston. "The outbreak of the American Civil War found him in the employ of the Southwestern Telegraph Company [(later Western Union)] in Alabama, and he was absorbed with it into the Confederacy. There was no getting away, and he was transferred to Nashville, Tenn."