Polk County was formed on January 13, 1846. It was named after President James K. Polk.
The first courthouse, a two-story structure, was built in 1846. Construction of a second courthouse was begun in 1858. Due to construction delays and the onset of the Civil War, the structure was not completed until 1866. The present courthouse was erected in 1900 in the Neo-Renaissance style. It was dedicated on October 31, 1906. In 1962, an extensive renovation was undertaken.
There were 149,112 households out of which 32.20% had children under the age of 18 living with them, 51.00% were married couples living together, 10.30% had a female householder with no husband present, and 35.20% were non-families. 28.10% of all households were made up of individuals and 8.60% had someone living alone who was 65 years of age or older. The average household size was 2.45 and the average family size was 3.04.
In the county the population was spread out with 25.70% under the age of 18, 9.40% from 18 to 24, 32.20% from 25 to 44, 21.50% from 45 to 64, and 11.10% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 94.20 males. For every 100 females age 18 and over, there were 90.70 males.
The median income for a household in the county was $46,116, and the median income for a family was $56,560. Males had a median income of $37,182 versus $28,000 for females. The per capita income for the county was $23,654. About 5.30% of families and 7.90% of the population were below the poverty line, including 9.70% of those under age 18 and 6.40% of those age 65 or over.