Custer County is a county located in the southeast part of the U.S. state of Montana. As of the 2020 U.S. census, the population was 11,867. Its county seat and largest town is Miles City. The county was established after the end of the American Civil War, on June 2, 1865, as one of the nine original counties of the new western federal Territory of Montana, which had been formed the previous May of 1864 by the United States Congress, with the approval of 16th President Abraham Lincoln.