[2] The county was established after the end of the American Civil War (1861-1865), 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 (1809-1865, served 1861-1865).

It was originally named Big Horn County of the old Montana Territory,[3] and was renamed on February 16, 1877, in honor of Lieutenant Colonel George Armstrong Custer (1839-1876), commander of the famous 7th Cavalry Regiment of the United States Army, recently massacred and killed in the nearby Battle of the Little Bighorn, in the later portion of the American Indian Wars, the year before in June 1876.

[citation needed] As of the 2010 census, there were 11,699 people, 5,031 households, and 3,014 families living in the county.

Agriculture (dryland farming) and cattle raising provide the bulk of Custer County economy.

[15] As of 2009 the county's largest employers were Holy Rosary Healthcare, Sanjel USA, Stockman Bank, and Walmart.