Mendon is a village in Adams County, Illinois, United States. The population was 872 at the 2020 census. It is part of the Quincy, IL–MO Micropolitan Statistical Area.

The Quincy–Hannibal, IL–MO Combined Statistical Area, as defined by the United States Census Bureau, is an area consisting of one county in Western Illinois and three counties in northeast Missouri, anchored by the cities of Quincy and Hannibal.