The Big Sioux River flows south-southeastward along the east line of Lincoln County. Lincoln County is on the eastern line of South Dakota. Its east boundary line abuts the west boundary line of the state of Iowa. The county terrain consists of low rolling hills. The county area is largely devoted to agriculture. The terrain slopes to the southeast, and drops off to the river valley along its east edge. The county has a total area of, of which is land and is water.
As of the 2000 United States Census, there were 24,131 people, 8,782 households, and 6,665 families in the county. The population density was 42 people per square mile. There were 9,131 housing units at an average density of 16 per square mile. The racial makeup of the county was 97.55% White, 0.34% Black or African American, 0.53% Native American, 0.46% Asian, 0.02% Pacific Islander, 0.29% from other races, and 0.81% from two or more races. 0.70% of the population were Hispanic or Latino of any race. There were 8,782 households out of which 41.00% had children under the age of 18 living with them, 66.50% were married couplesliving together, 6.70% had a female householder with no husband present, and 24.10% were non-families. 19.50% of all households were made up of individuals and 8.00% had someone living alone who was 65 years of age or older. The average household size was 2.72 and the average family size was 3.14. The county population contained 29.70% under the age of 18, 7.60% from 18 to 24, 31.90% from 25 to 44, 20.40% from 45 to 64, and 10.40% who were 65 years of age or older. The median agewas 34 years. For every 100 females, there were 99.80 males. For every 100 females age 18 and over, there were 97.50 males. The median income for a household in the county was $48,338, and the median income for a family was $55,401. Males had a median income of $34,486 versus $24,133 for females. The per capita income for the county was $22,304. About 3.20% of families and 4.40% of the population were below the poverty line, including 4.70% of those under age 18 and 7.80% of those age 65 or over.
2010 census
As of the 2010 United States Census, there were 44,828 people, 16,649 households, and 12,287 families in the county. The population density was. There were 17,875 housing units at an average density of. The racial makeup of the county was 96.1% white, 1.0% Asian, 0.7% black or African American, 0.5% American Indian, 0.3% from other races, and 1.4% from two or more races. Those of Hispanic or Latino origin made up 1.2% of the population. In terms of ancestry, 47.1% were German, 20.9% were Norwegian, 11.7% were Irish, 7.2% were Dutch, 5.2% were English, and 3.9% were American. Of the 16,649 households, 41.0% had children under the age of 18 living with them, 63.3% were married couples living together, 7.3% had a female householder with no husband present, 26.2% were non-families, and 20.2% of all households were made up of individuals. The average household size was 2.68 and the average family size was 3.11. The median age was 32.8 years. The median income for a household in the county was $67,365 and the median income for a family was $75,231. Males had a median income of $43,537 versus $34,715 for females. The per capita income for the county was $33,261. About 2.7% of families and 4.3% of the population were below the poverty line, including 4.9% of those under age 18 and 7.5% of those age 65 or over.
Lincoln County voters have been reliably Republican for decades. In no national election since 1964 has the county selected a Democratic Party candidate.
