During the American Civil War, Joslin served with the 25th Wisconsin Volunteer Infantry Regiment of the Union Army. Battles he participated in include the Battle of Resaca, the Battle of Dallas, the Battle of Kennesaw Mountain, the Battle of Atlanta, the Battle of Decatur, the Battle of Rivers' Bridge and the Battle of Bentonville. Additionally, Joslin took part in the Meridian Expedition and Sherman's March to the Sea. He was brevetted a colonel.