According to the U.S. Census Bureau, the county has a total area of 524 square miles (1,360 km2), of which 520 square miles (1,300 km2) is land and 4.1 square miles (11 km2) (0.8%) is water. Its largest cities are situated along the Fox River.
In recent years, average temperatures in the county seat of Geneva have ranged from a low of 10 °F (−12 °C) in January to a high of 84 °F (29 °C) in July, although a record low of −26 °F (−32 °C) was recorded in January 1985 and a record high of 111 °F (44 °C) was recorded in July 1936. Average monthly precipitation ranged from 1.52 inches (39 mm) in February to 4.39 inches (112 mm) in July.
There were 133,901 households out of which 41.60% had children under the age of 18 living with them, 61.20% were married couples living together, 10.00% had a female householder with no husband present, and 24.20% were non-families. 19.60% of all households were made up of individuals and 6.70% had someone living alone who was 65 years of age or older. The average household size was 2.97 and the average family size was 3.43.
In the county, the population was spread out with 30.30% under the age of 18, 9.10% from 18 to 24, 31.90% from 25 to 44, 20.40% from 45 to 64, and 8.40% who were 65 years of age or older. The median age was 32 years. For every 100 females there were 101.20 males. For every 100 females age 18 and over, there were 98.80 males.
The median income for a household in the county was $59,351, and the median income for a family was $66,558. Males had a median income of $45,787 versus $30,013 for females. The per capita income for the county was $24,315. About 6.60% of families and 8.40% of the population were below the poverty line.
Forstall, Richard L. (editor) (1996). Population of states and counties of the United States: 1790 to 1990 : from the twenty-one decennial censuses. United States Department of Commerce, Bureau of the Census, Population Division. ISBN0-934213-48-8.