Born in Sandusky, Ohio, he fought at Gettysburg under George Armstrong Custer, developed the Mann Boudoir Car, a railroad sleeping car, and published the Mobile Register, The Smart Set, and Town Topics. The credibility of the latter was undermined by Mann's tacit admission in civil court to allowing robber barons to purchase immunity from coverage in the paper. Following the war, he was a member of the Military Order of the Loyal Legion of the United States.