After the 1832 Black Hawk War, land-hungry New Englanders and western Virginians pushed out along Indian trails radiating away from Chicago in search of farm sites. To the north and northwest, they found a rolling, well-watered prairie interspersed with large oak groves. Finding access to Cook County government difficult, they petitioned for the formation of a more local county seat. The Illinois legislature approved the petition in 1836 and named the new county McHenry, after a leader of volunteers in the area during the Black Hawk War.
McHenry County stretched west from Lake Michigan to present-day Boone County and north from Kane County to the Wisconsin state line. Since the Fox River flowed through the approximate center of the county and was easily fordable at the present site of McHenry, that village became the county seat.
The eastern side of the new county was settled earliest as most newcomers established homes along the Green Bay Trail. Farmers and speculators there found trips around numerous glacial lakes and across wet prairies to McHenry too time consuming. In 1839 voters in eastern McHenry County were granted the right to form a new county (Lake County.)
While the new boundary was supposed to be the Fox River, a survey put almost all of the river's valley in McHenry County. Although a courthouse already stood in the village of McHenry, a more central location was demanded. In 1844 the county seat was removed to a midcounty location platted as Centerville and renamed Woodstock in 1845.
In early decades of the county's history, poor roads left residents isolated from markets. The county's most economically successful settlements (Crystal Lake, Marengo, and Richmond) were located along well-established Indian trails from Chicago
The introduction of railroads brought great changes. The Chicago & Northwestern Railroad blanketed the county. One line pushed west through and Marengo by early 1851. Two other lines were established in 1854. One ran diagonally across the county from the Fox River at Cary, reaching Harvard by 1855. The other ran north from Algonquin through the eastern McHenry County beyond Richmond. A fourth line graced the northern tier of townships by 1861. After 1900, a "Milwaukee Road" line would cross the northeastern corner of the county. Communities built in partnership with the railroad such as Harvard, Nunda (later Crystal Lake), and Cary prospered while non-rail communities such as Franklinville, Coral, Ostend, and Barreville faded from existence.
Many Irish railroad laborers stayed and became farmers, cheaply shipping their produce to market. Chicago's demand for fresh milk products along with Gail Borden's milk condensing plants convinced farmers to try dairying. Railroad milk stations and cheese "factories" quickly appeared in all the county's townships. In modern times the arts and recreation add to the high quality of life enjoyed in McHenry County while maintaining the history and culture of its agricultural past.