According to the US Census Bureau, the county has a total area of, of which is land and is water.
Climate and weather
In recent years, average temperatures in the county seat of Carrollton have ranged from a low of in January to a high of in July, although a record low of was recorded in January 1912 and a record high of was recorded in July 1934. Average monthly precipitation ranged from in January to in May.
As of the 2010 United States Census, there were 13,886 people, 5,570 households, and 3,777 families living in the county. The population density was. There were 6,389 housing units at an average density of. The racial makeup of the county was 97.9% white, 0.9% black or African American, 0.2% American Indian, 0.1% Asian, 0.3% from other races, and 0.7% from two or more races. Those of Hispanic or Latino origin made up 0.8% of the population. In terms of ancestry, 30.7% were German, 14.7% were Irish, 13.3% were English, and 12.1% were American. Of the 5,570 households, 30.9% had children under the age of 18 living with them, 53.0% were married couplesliving together, 9.4% had a female householder with no husband present, 32.2% were non-families, and 27.8% of all households were made up of individuals. The average household size was 2.44 and the average family size was 2.95. The median age was 41.6 years. The median income for a household in the county was $41,450 and the median income for a family was $52,049. Males had a median income of $38,185 versus $27,231 for females. The per capita income for the county was $22,107. About 11.8% of families and 15.1% of the population were below the poverty line, including 22.1% of those under age 18 and 10.0% of those age 65 or over.
Greene County was reliably Democratic from the beginning through 1948; only one Republican Party nominee carried the county vote during that period. Since then it has usually voted for the Republican nominee. As one of the most northerly “southern” counties in Illinois, Greene County was rock-ribbed Democratic for the seventy years after the Civil War, which it opposed as a “Yankee” war. Not until considerable anti-Catholic sentiment against Al Smith turned many voters to Herbert Hoover did the county support a Republican presidential nominee. However, with the coming of World War II, opposition to American involvement led to gains for Wendell Willkie and Thomas E. Dewey, although apart from the 1960 election – also influenced by Catholicism – Greene was a bellwether county throughout the period from 1928 to 2004. In recent years, like most of downstate Illinois, opposition to the Democratic Party's liberal views on social issues, along with economic problems, has caused very rapid swings toward the Republican Party. Hillary Clinton’s 2016 tally of 21.6 percent of the county’s vote is fifteen percent worse than any Democrat before 2010.
