In recent years, average temperatures in the county seat of Quincy have ranged from a low of 16 °F (−9 °C) in January to a high of 88 °F (31 °C) in July, although a record low of −21 °F (−29 °C) was recorded in January 1979 and a record high of 105 °F (41 °C) was recorded in July 2005. Average monthly precipitation ranged from 1.36 inches (35 mm) in January to 4.61 inches (117 mm) in May.
As of the 2010 United States Census, there were 67,103 people, 27,375 households, and 17,677 families residing in the county. The population density was 78.5 inhabitants per square mile (30.3/km2). There were 29,842 housing units at an average density of 34.9 per square mile (13.5/km2). The racial makeup of the county was 93.7% white, 3.5% black or African American, 0.7% Asian, 0.2% American Indian, 0.4% from other races, and 1.6% from two or more races. Those of Hispanic or Latino origin made up 1.2% of the population. In terms of ancestry, 43.5% were German, 13.1% were Irish, 10.7% were American, and 8.7% were English.
Of the 27,375 households, 29.9% had children under the age of 18 living with them, 50.0% were married couples living together, 10.7% had a female householder with no husband present, 35.4% were non-families, and 30.1% of all households were made up of individuals. The average household size was 2.37 and the average family size was 2.94. The median age was 40.7 years.
The median income for a household in the county was $43,824 and the median income for a family was $55,791. Males had a median income of $38,830 versus $29,371 for females. The per capita income for the county was $24,308. About 8.3% of families and 12.3% of the population were below the poverty line, including 17.5% of those under age 18 and 8.1% of those age 65 or over.
Presidential Election Results (Adams County, IL):1960–present (U.S. winner is represented by (W), IL winner is represented by (I)), Adams County winner is represented by italicized number of votes received in the county