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 census of 2000, there were 35,535 people, 12,754 households, and 9,221 families residing in the county. The population density was 75 people per square mile (29/km²). There were 13,805 housing units at an average density of 29 per square mile (11/km²). The racial makeup of the county was 94.19% White, 3.91% Black or African American, 0.33% Asian, 0.16% Native American,0.03% Pacific Islander, 0.84% from other races, and 0.54% from two or more races. 1.60% of the population were Hispanic or Latino of any race. 62.1% were of German and 11.0% American ancestry according to Census 2000. 96.8% spoke English, 2.0% Spanish and 1.0% German as their first language.
There were 12,754 households out of which 35.10% had children under the age of 18 living with them, 60.10% were married couples living together, 8.40% had a female householder with no husband present, and 27.70% were non-families. 24.20% of all households were made up of individuals and 11.90% had someone living alone who was 65 years of age or older. The average household size was 2.60 and the average family size was 3.10.
In the county the population was spread out with 24.90% under the age of 18, 9.30% from 18 to 24, 30.20% from 25 to 44, 21.30% from 45 to 64, and 14.40% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 106.60 males. For every 100 females age 18 and over, there were 106.80 males.
The median income for a household in the county was $44,618, and the median income for a family was $52,580. Males had a median income of $36,035 versus $23,506 for females. The per capita income for the county was $19,109. About 4.60% of families and 6.40% of the population were below the poverty line, including 7.80% of those under age 18 and 6.00% of those age 65 or over.