Union county  was originally part of Johnson  county. It was named  "Union County" for the federal Union. This is because it became a county the same year Illinois joined the Union in 1818.
   The Union County courthouse is located in Jonesboro.  Jonesboro is located in the southwestern corner of Union County. 
   Some of the largest cities in Union County are Millcreek, Wolf Lake, Dongola, Jonesboro, Mill Creek, Alto Pass, Anna, Cobden.