The James H. Mills papers contain one reminiscence by Mills about The Montana Post, twenty diaries written between 1862 and 1904, and one account volume from Company G of the 40th Pennsylvania Infantry. The diaries cover Mills' Civil War activities, travel to Montana, newspaper publishing, Republican politics, the elections of 1892 and 1896, the Pullman railroad car strike of 1894, family life, social activities, and territorial government.