McHenry is a city in
McHenry County,
Illinois,
United States. The 2010 U.S. census reported its population at 26,992. McHenry was at one time the county seat of McHenry County, which once included adjoining Lake County to the east. McHenry took its name from McHenry County, which was named for Major
William McHenry, an Indian fighter.