2008 - 318,641 2000 - 260,077 1990 - 183,241 1980 - 147,897 1970 - 111,555 1960 - 84,210 1950 - 50,656 1940 - 37,311 1930 - 35,079 1920 - 33,164 1910 - 32,509 1900 - 29,759 1890 - 26,114 1880 - 24,908 1870 - 23,762 1860 - 22,089 1850 - 14,978 1840 - 2,578
As of the census of 2000, there were 260,077 people, 89,403 households, and 69,287 families residing in the county. The population density was 431 people per square mile (166/km²). There were 92,908 housing units at an average density of 154 per square mile (59/km²). The racial makeup of the county was 93.91% White, 0.59% Black or African American, 0.17% Native American, 1.45% Asian, 0.02% Pacific Islander, 2.77% from other races, and 1.08% from two or more races. 7.54% of the population were Hispanic or Latino of any race. 28.5% were of German, 12.2% Irish, 9.8% Polish, 8.8% Italian and 5.6% English ancestry according to Census 2000. 89.3% spoke English, 6.4% Spanish and 1.0% Polish as their first language. The population of the Polish population has doubled within the last several years.
There were 89,403 households out of which 42.90% had children under the age of 18 living with them, 66.50% were married couples living together, 7.60% had a female householder with no husband present, and 22.50% were non-families. 18.00% of all households were made up of individuals and 6.10% had someone living alone who was 65 years of age or older. The average household size was 2.89 and the average family size was 3.31.
In the county the population was spread out with 30.20% under the age of 18, 7.10% from 18 to 24, 33.50% from 25 to 44, 21.30% from 45 to 64, and 8.00% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 100.70 males. For every 100 females age 18 and over, there were 98.00 males.
The median income for a household in the county was $64,826, and the median income for a family was $71,553. Males had a median income of $50,479 versus $31,141 for females. The per capita income for the county was $26,476. About 2.50% of families and 3.70% of the population were below the poverty line, including 3.80% of those under age 18 and 3.60% of those age 65 or over.
Several state highways run through McHenry County, including U.S. Route 20, U.S. Route 14, U.S. Route 12, Illinois Route 23, Illinois Route 173, Illinois Route 120, Illinois Route 176, Illinois Route 47, Illinois Route 31, and Illinois Route 62. Highly traveled county highways include Randall Road, Algonquin Road, and Walkup Road.
McHenry County is also the most populous county in the United States without direct access to an Interstate within its borders. As a result, most residents use nearby Interstate 90 (Jane Addams Memorial Tollway), a couple miles south in Kane County, to access the interstate highway system.
McHenry County is also served by Metra Rail, which provides daily commuter service to and from downtown Chicago. There are seven stations in the county, all of them located along the Union Pacific Chicago & Northwest line. The county is also served by Pace, which provides three regular bus routes, and dial-a-ride service for several communities.
Lake in the Hills Airport offers general aviation service, as do Dacy Airport and Galt Airport, in the northern part of the county. Chicago-O'Hare International Airport is approximately 30-40 miles from the county, while Milwaukee's General Mitchell International Airport is approximately 50-60 miles. Chicago Rockford International Airport, a busy cargo airport with limited, scheduled passenger service is located approximately 42 miles west of the central part of the county (Woodstock).