According to the U.S. Census Bureau, the county has a total area of 871 square miles (2,260 km2), of which 855 square miles (2,210 km2) is land and 16 square miles (41 km2) (1.9%) is water.[5]
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.[4]
U.S. Decennial Census[7] 1790–1960[8] 1900–1990[9] 1990–2000[10] 2010–2013[1]
As of the 2020 United States Census, there were 65,737 people, 27,199 households, and 17,398 families residing in the county.[11] The population density was 75.5 inhabitants per square mile (29.2/km2). There were 30,235 housing units at an average density of 34.7 per square mile (13.4/km2). The racial makeup of the county was 89.5% white, 3.8% black or African American, 0.8% Asian, 0.2% American Indian, 0.8% from other races, and 4.9% from two or more races. Those of Hispanic or Latino origin made up 2.0% of the population. The most common reported ancestries were, German (35.9%), Irish (11.1%), English (8.1%), and American (7.4%).
Of the 27,199 households, 25.1% had children under the age of 18 living with them, 50.2% were married couples living together, 9.8% had a female householder with no husband present, 36.0% were non-families. 30.5% of all households were made up of individuals, and 15.4% had someone living alone who was 65 years of age or older. The average household size was 2.36 and the average family size was 2.95.
In the county, the population was spread out, with 22.6% under the age of 18, 7.5% from 18 to 24, 23.7% from 25 to 44, 25.9% from 45 to 64, and 20.2% who were 65 years of age or older. The median age was 41.4 years. For every 100 females, there were 96.4 males. For every 100 females age 18 and over, there were 93.5 males.
The median income for a household in the county was $55,052 and the median income for a family was $72,091. Males had a median income of $41,852 versus $29,404 for females. The per capita income for the county was $31,035. About 9.4% of families and 12.5% of the population were below the poverty line, including 17.7% of those under age 18 and 9.9% of those age 65 or over.[12]
The county is part of the historic belt of German settlement extending into the Missouri Rhineland. Since it was antagonistic to the Yankee northeast of Illinois, it voted solidly Democratic until 1892. After being a swing county in the first half of the twentieth century, Adams County has been a Republican stronghold. It has gone Republican in all but four presidential elections since 1920, all but one of which was a 400-vote Democratic landslide. The county last supported a Democrat in 1964, when it voted for Lyndon Johnson. The county regularly supports the Republicans at the state level as well; it has not supported a Democrat for Governor of Illinois since Adlai Stevenson II in 1948. Additionally, five of the six countywide elected officials in Adams County are Republicans, with a Democrat holding the position of Circuit Clerk.[16][17] Notably, while it voted for Barack Obama in his 2004 Senate campaign, he lost it by wide margins in both of his presidential bids.
![When it was created, Hancock County was temporarily attached to Adams until it could organize a county government.[3]](File:Adams_County_Illinois_1825.png)
