The Old Courthouse was designed in 1855 by the first professional architect of Chicago, John Mills Van Osdel (1811-1892).

He was one of the most prolific architects in Chicago and designed many of the most important buildings in early Chicago including the first mayor’s mansion, several successive Chicago city halls and Cook County courthouses. He designed the city’s first brick theater and the city’s first high school and built numerous famous hotels including The Palmer House. In fact, Van Osdel actually sold Potter Palmer the land for the first Palmer House and designed and built three of the four Palmer House Hotels for him. Van Osdel also designed several of the first opening bridges over the Chicago River as well as the first steamships and the first grain elevators ever built in the city. He received the first US patent ever issued in the city of Chicago for his design of windmill powered pumps for lifting water in the Illinois and Michigan Canal.

Abraham Lincoln learned he had been nominated for the Presidency of the United States in his suite at the Van Osdel-designed Briggs House Hotel, and the Illinois Republican Party headquarters were in Van Osdel’s Tremont House Hotel. Van Osdel also designed the catafalque that Abraham Lincoln’s coffin was placed on in the rotunda of the Van Osdel-designed Cook County courthouse.

A fire destroyed a large portion of the Woodstock Square on the same day as the Great Chicago Fire of 1871.  While the Old Courthouse was spared from the Woodstock fire, the Great Chicago Fire destroyed much of Chicago, including most of Van Osdel’s early work. Within just a few years of the fire, Van Osdel rebuilt the city by designing buildings spanning more than a mile and a half of Chicago street frontage.  This included about a half mile along Lake Street that was then the main street of the city. However, almost all of these buildings have since been lost to demolition.  This makes the Old McHenry County Courthouse one of the few surviving examples of Van Osdel’s work, along with the Illinois Executive Mansion in Springfield and the venerable “Old Main” building at the University of Arkansas.

Van Osdel was responsible for the passage of the first building code in Chicago and was instrumental in raising the grade of the swampy city. He was involved in the formation of the University of Illinois and built the first building on the Champaign campus. He was also instrumental in the establishment of UIC, The University of Illinois at Chicago. The last building he designed was the first modern building in the world to reach 13 stories. As prolific as Van Osdel was, it is remarkable that the Old McHenry County Courthouse is one of the last examples of his work still standing.