McHenry County is a county located in the U.S. state of Illinois. According to the 2010 census, it had a population of 308,760, making it the sixth-most populous county in Illinois. Its county seat is Woodstock.
McHenry County was formed in 1836 out of Cook and LaSalle counties. The county was named for Major William McHenry, an Indian fighter who died in Vandalia in 1835. McHenry County originally stretched all the way east to Lake Michigan, with the county seat centrally in McHenry, but in 1839, the eastern townships of the county were carved out to form Lake County.