In the Nebraska license plate system, Polk County is represented by the prefix 41 (it had the forty-first-largest number of vehicles registered in the county when the license plate system was established in 1922).
As of the census of 2000, there were 5,639 people, 2,259 households, and 1,570 families residing in the county. The population density was 5/km² (13/mi²). There were 2,717 housing units at an average density of 2/km² (6/mi²). The racial makeup of the county was 98.92% White, 0.02% Black or African American, 0.28% Native American, 0.09% Asian, 0.28% from other races, and 0.41% from two or more races. 1.08% of the population were Hispanic or Latino of any race.
There were 2,259 households out of which 29.90% had children under the age of 18 living with them, 62.90% were married couples living together, 4.10% had a female householder with no husband present, and 30.50% were non-families. 27.60% of all households were made up of individuals and 15.40% had someone living alone who was 65 years of age or older. The average household size was 2.43 and the average family size was 2.97.
In the county the population was spread out with 25.10% under the age of 18, 6.00% from 18 to 24, 24.40% from 25 to 44, 23.10% from 45 to 64, and 21.40% who were 65 years of age or older. The median age was 42 years. For every 100 females there were 100.50 males. For every 100 females age 18 and over, there were 95.70 males.
The median income for a household in the county was $37,819, and the median income for a family was $45,081. Males had a median income of $30,286 versus $19,595 for females. The per capita income for the county was $17,934. About 4.40% of families and 5.80% of the population were below the poverty line, including 7.20% of those under age 18 and 4.70% of those age 65 or over.