McHenry is a city in McHenry County, Illinois, United States. It is located about 55 miles northwest of Chicago. Per the 2020 census, the population was 27,135. 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 the county, which was named for Major William McHenry, a prominent US Army officer in the Black Hawk War.