McHenry County was formed in 1836 out of Cook and LaSalle counties. The county was named for Major William McHenry, a member of the Illinois Militia during Tecumseh's War, a major during the Blackhawk War in 1832, and a member of the Illinois House of Representatives and Senate. He died in Vandalia in 1835.

According to the 2020 Census, it had a population of 310,229,[2] making it the sixth-most populous county in Illinois. Its county seat is Woodstock.[3] McHenry County is one of the five collar counties of 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.