On September 24 , 1868 , Robert Lincoln married Mary Eunice Harlan ( September 25 , 1846 -- March 31 , 1937 ) . On September 24 , 1868 , Robert Lincoln married the former Mary Eunice Harlan ( September 25 , 1846 - March 31 , 1937 ) , the daughter of Senator James Harlan and Ann Eliza Peck of Mount Pleasant , Iowa .