In 1862, Sladen joined the Union Army for the American Civil War, enlisting as a private in Company A, 33rd Massachusetts Infantry Regiment. He took part in several of the regiment's engagements, including the Battle of Chancellorsville and Battle of Gettysburg. Sladen took part in the May 13-15, 1864 Battle of Resaca. While serving as a clerk at the regimental headquarters, Sladen left his post to voluntarily join the battle. His personal example inspired his comrades at a critical moment in the fight, and they rallied to defeat their Confederate opponents. In 1895, he received the Medal of Honor in acknowledgement of his Civil War heroism.