The county was established on March 1, 1818, and is named for General George Rogers Clark who fought Indians in Ohio in the early Nineteenth century. Its county seat is Springfield.
2007 140,477 2000 144,742 1990 147,548 1980 150,236 1970 157,115 1960 131,440 1950 111,661 1940 95,647 1930 90,936 1920 80,728 1910 66,435 1900 58,939 1890 52,277 1880 41,948 1870 32,070 1860 25,300 1850 22,178 1840 16,882 1830 13,114 1820 9,533
As of the census of 2000, there were 144,742 people, 56,648 households, and 39,370 families residing in the county. The population density was 362 people per square mile (140/km²). There were 61,056 housing units at an average density of 153 per square mile (59/km²). The racial makeup of the county was 88.12% White, 8.95% Black or African American, 0.28% Native American, 0.53% Asian, 0.02% Pacific Islander, 0.53% from other races, and 1.58% from two or more races. 1.17% of the population were Hispanic or Latino of any race. 23.8% were of German, 21.6% American, 10.4% Irish and 8.7% English ancestry according to Census 2000.
There were 56,648 households out of which 31.40% had children under the age of 18 living with them, 52.60% were married couples living together, 12.80% had a female householder with no husband present, and 30.50% were non-families. 26.00% of all households were made up of individuals and 11.10% had someone living alone who was 65 years of age or older. The average household size was 2.49 and the average family size was 2.97.
In the county the population was spread out with 25.10% under the age of 18, 9.10% from 18 to 24, 26.80% from 25 to 44, 24.30% from 45 to 64, and 14.70% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 92.50 males. For every 100 females age 18 and over, there were 88.60 males.
The median income for a household in the county was $40,340, and the median income for a family was $48,259. Males had a median income of $37,157 versus $24,688 for females. The per capita income for the county was $19,501. About 7.90% of families and 10.70% of the population were below the poverty line, including 14.90% of those under age 18 and 8.20% of those age 65 or over.