Arapahoe County, Colorado is the state’s first county and the third largest with a total population of 637,068 and an unincorporated population of 83,764. The County spans 805 square miles and is a land of contrast. The western part of the county is mostly urban with residential, retail, office and industrial areas, while the eastern portion is relatively rural. Arapahoe County has one of the lowest mill levies in the Front Range.