The 1857 Italianate Old Courthouse was constructed to closely resemble the 1853 Cook County Courthouse (which was eventually destroyed during the Great Chicago Fire). The structure was designed by John M. Van Osdel, the first licensed architect in Chicago. The adjoining structure, the Sheriff's House and Jail was built in 1887. Until 1973 McHenry County government office were located in the Old Courthouse. When the government moved out, the building was threatened with demolition, as it stood on prime commercial property. On November 1, 1974, it was recognized by the National Park Service on the National Register of Historic Places.

The Old McHenry County Courthouse faces the Woodstock public square. The Italianate structure was built out of brick with limestone trim. With four identical, 20-by-44-square-foot wings around a 44-square-foot core, the building resembles a Greek cross. The building stands two stories tall with a brick basement. A cupola is found at the center of the cross. A bay was later added to the southern wing, and small additions were made from the judge's chamber and a vault. The southwest corner has a one-story addition. The jail was originally found in the basement, but was moved to a new building along the north wall in the 1870s.