McHenry is a city in McHenry County, Illinois, United States. It is located about 50 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.

McHenry County is a county located in the U.S. state of Illinois. According to the 2020 Census, it had a population of 310,229, making it the sixth-most populous county in Illinois. Its county seat is Woodstock. McHenry County is one of the five Illinois collar counties in the Chicago-Naperville-Elgin, IL-IN-WI Metropolitan Statistical Area. Long known as a center of recreation along with agriculture in the western portion, it has more recently experienced rapid rates of suburbanization, exurbanization and urbanization, but the western portions of the county remain primarily agricultural and rural.