McHenry is a city in McHenry County, Illinois, United States. It is located about 46 miles northwest of Chicago. Per the 2020 census, the population was 27,135. McHenry was at one time the county seat of McHenry County, which once included adjoining Lake County to the east. McHenry took its name from the county, which was named for Major William McHenry, a prominent US Army officer in the Black Hawk War.

It rests at an elevation of 797 feet and the Fox River flows through the eastern portion of the city. It is surrounded by natural lakes and streams, grassy moraine hills, gravel banks and shallow nutrient-rich peat bogs, remnants of receding glaciers from the last ice age. Moraine Hills State Park, Glacial Park Conservation Area, and Volo Bog State Natural Area preserve some of these natural features.

In the 1830s various settlers arrived in the area and created the foundation for McHenry. Some of the family names can still be seen today: McCullom, McLean, Boone, and Wheeler.

George Gage came to the area in 1835 and purchased a plot of land west of the Fox River. This area eventually came to be known as Gagetown. In 1854, George Gage played an instrumental role in bringing the railroad to the city. Gagetown was eventually renamed to West McHenry.

In 1851, a dam was built on Boone Creek which created Mill Pond. The pond encompassed an area between today's rail road tracks and Route 31 and north of Waukegan Road.

In 1860, the historical Count's House was completed.

On August 4, 1875, the first issue of the city newspaper, the McHenry Plaindealer, was published by Jay Van Slyke.

In 1880, the first bridge in McHenry was built at Pearl Street.