Missouri folklorist Margot Ford McMillen wrote that early settlers were attracted by Clark County's good and inexpensive agricultural land. One section was called "Bit Nation" because land was sold there for just twelve and one-half cents ("one bit" of a Spanish dollar) an acre.
According to the 2000 U.S. Census, the county has a total area of 511.90 square miles (1,325.8 km2), of which 507.31 square miles (1,313.9 km2) (or 99.10%) is land and 4.59 square miles (11.9 km2) (or 0.90%) is water.
There were 2,966 households out of which 30.30% had children under the age of 18 living with them, 58.70% were married couples living together, 7.00% had a female householder with no husband present, and 29.90% were non-families. 26.40% of all households were made up of individuals and 13.60% had someone living alone who was 65 years of age or older. The average household size was 2.46 and the average family size was 2.95.
In the county the population was spread out with 25.00% under the age of 18, 7.80% from 18 to 24, 25.50% from 25 to 44, 25.00% from 45 to 64, and 16.70% who were 65 years of age or older. The median age was 39 years. For every 100 females there were 97.60 males. For every 100 females age 18 and over, there were 94.00 males.
The median income for a household in the county was $29,457, and the median income for a family was $36,270. Males had a median income of $27,279 versus $19,917 for females. The per capita income for the county was $15,988. About 10.80% of families and 14.10% of the population were below the poverty line, including 19.70% of those under age 18 and 12.70% of those age 65 or over.