Having received promotion to corporal, in 1864 Sladen was commissioned as a first lieutenant of Volunteers and assigned to the 14th United States Colored Infantry Regiment. Detailed as an aide-de-camp to General Oliver Otis Howard, he remained in the Union Army until 1866. Sladen took part in the September 1864 Battle of Jonesborough, for which he received regular army brevet promotions to first lieutenant and captain.