Jefferson County is a county located in the southern part of the U.S. state of Illinois. According to the 2020 census, it has a population of 37,113. The county seat is Mount Vernon. Jefferson County contains the Mount Vernon, Illinois micropolitan area. It is located in the southern portion known locally as "Little Egypt". The western border of the county adjoins the Greater St. Louis consolidated metropolitan statistical area, the Metro-East region, and the St. Louis commuter region and market and viewing area.