There are 5 US Post Offices in Atchison County serving a total of 16,516 residents equating to 3,303 residents per post office. It's estimated that approximately 26,361 packages are handled across the 5 county post offices annually. Atchison County (county code AT) is a county located in northeastern Kansas, in the Central United States. As of the 2010 census, the county population was 16,924. Its county seat and most populous city is Atchison. The county is named in honor of David Rice Atchison, a United States Senator from Missouri.