In recent years, average temperatures in the county seat of Belvidere have ranged from a low of 11 °F (−12 °C) in January to a high of 85 °F (29 °C) in July, although a record low of −29 °F (−34 °C) was recorded in January 1979 and a record high of 109 °F (43 °C) was recorded in July 1936. Average monthly precipitation ranged from 1.29 inches (33 mm) in February to 4.56 inches (116 mm) in June.
As of the 2010 United States Census, there were 54,165 people, 18,505 households, and 14,273 families residing in the county. The population density was 193.0 inhabitants per square mile (74.5/km2). There were 19,970 housing units at an average density of 71.1 per square mile (27.5/km2). The racial makeup of the county was 84.4% white, 2.0% black or African American, 1.3% Asian, 0.4% American Indian, 9.3% from other races, and 2.6% from two or more races. Those of Hispanic or Latino origin made up 20.2% of the population. In terms of ancestry, 25.6% were German, 13.7% were Irish, 9.2% were English, 8.6% were American, 7.7% were Italian, 7.1% were Swedish, and 5.0% were Polish.
Of the 18,505 households, 41.5% had children under the age of 18 living with them, 61.6% were married couples living together, 10.2% had a female householder with no husband present, 22.9% were non-families, and 18.9% of all households were made up of individuals. The average household size was 2.91 and the average family size was 3.32. The median age was 36.8 years.
The median income for a household in the county was $61,210 and the median income for a family was $69,380. Males had a median income of $53,581 versus $34,651 for females. The per capita income for the county was $26,105. About 7.9% of families and 10.4% of the population were below the poverty line, including 15.3% of those under age 18 and 5.0% of those age 65 or over.
^The Past and Present of Boone County, Illinois: Containing a History of the County, Its Cities, Towns, Etc., a Biographical Directory of Its Citizens, War Record of Its Volunteers in the Late Rebellion, Portraits of Early Settlers and Prominent Men, General and Local Statistics, History of the Northwest, History of Illinois, Constitution of the United States, Map of Boone County, Miscellaneous Matters, Etc., Etc by H.F. Kett & Company, 1877 page 226