According to the U.S. Census Bureau, the county has a total area of 588 square miles (1,520 km2), of which 581 square miles (1,500 km2) is land and 6.7 square miles (17 km2) (1.1%) is water. Native vegetation based on NRCS soils information shows:
In recent years, average temperatures in the county seat of Montevideo have ranged from a low of 2 °F (−17 °C) in January to a high of 83 °F (28 °C) in July, although a record low of −37 °F (−38 °C) was recorded in January 1970 and a record high of 110 °F (43 °C) was recorded in July 1988. Average monthly precipitation ranged from 0.86 inches (22 mm) in December to 4.24 inches (108 mm) in June.
There were 5,361 households, out of which 31.20% had children under the age of 18 living with them, 57.00% were married couples living together, 6.60% had a female householder with no husband present, and 32.90% were non-families. 29.50% of all households were made up of individuals and 15.70% had someone living alone who was 65 years of age or older. The average household size was 2.39 and the average family size was 2.96.
In the county the population was spread out with 25.40% under the age of 18, 7.10% from 18 to 24, 24.50% from 25 to 44, 23.00% from 45 to 64, and 20.00% who were 65 years of age or older. The median age was 40 years. For every 100 females there were 94.80 males. For every 100 females age 18 and over, there were 91.30 males.
The median income for a household in the county was $35,582, and the median income for a family was $45,160. Males had a median income of $30,556 versus $20,384 for females. The per capita income for the county was $18,039. About 4.80% of families and 8.60% of the population were below the poverty line, including 9.80% of those under age 18 and 9.30% of those age 65 or over.