In recent years, average temperatures in the county seat of Carlyle have ranged from a low of 19 °F (−7 °C) in January to a high of 88 °F (31 °C) in July, although a record low of −22 °F (−30 °C) was recorded in January 1994 and a record high of 104 °F (40 °C) was recorded in July 1980. Average monthly precipitation ranged from 2.17 inches (55 mm) in January to 4.44 inches (113 mm) in June.
As of the 2010 United States Census, there were 37,762 people, 14,005 households, and 9,760 families residing in the county. The population density was 79.7 inhabitants per square mile (30.8/km2). There were 15,311 housing units at an average density of 32.3 per square mile (12.5/km2). The racial makeup of the county was 93.4% white, 3.5% black or African American, 0.4% Asian, 0.2% American Indian, 1.2% from other races, and 1.1% from two or more races. Those of Hispanic or Latino origin made up 2.8% of the population. In terms of ancestry, 54.8% were German, 9.8% were Irish, 5.8% were English, and 5.6% were American.
Of the 14,005 households, 32.7% had children under the age of 18 living with them, 56.5% were married couples living together, 8.7% had a female householder with no husband present, 30.3% were non-families, and 25.1% of all households were made up of individuals. The average household size was 2.55 and the average family size was 3.02. The median age was 39.3 years.
The median income for a household in the county was $55,278 and the median income for a family was $66,682. Males had a median income of $45,119 versus $34,051 for females. The per capita income for the county was $25,392. About 5.2% of families and 7.8% of the population were below the poverty line, including 9.2% of those under age 18 and 5.7% of those age 65 or over.