Front of the former Allen County Jail, located at 204 N. Jefferson Street in Iola, Kansas, United States. Built in 1869, the jail is listed on the National Register of Historic Places. – Gordon Huggins, CC BY 2.0 – Wikimedia Commons Allen County is located in Kansas. About 12,526 people live here (as of 2020). Between 2010 and 2020, the population declined from 13,371 to 12,526. The area covers about 1,308.0 square kilometers. The region is divided into 9 Municipalities. The region was founded in 1855. The name originates from William Allen (United States Representative from Ohio) Allen County is county in Kansas, United States. More information at www.AllenCounty.org.