The county is home to historical events at Mendota that defined the state's future which included providing materials for the construction of Fort Snelling across the river and the signing of the Treaty of Traverse des Sioux which ceded land from native Dakota for the Minnesota Territory. The county's history was initially tied to the confluence of the Mississippi and Minnesota Rivers, both strategically important for United States expansion and as the convergence of the Dakota and Anishinaabe nations who regarded the site as sacred. Influence shifted westward during the post World War II settlement boom when Interstate 35 connected the western half of the county to Minneapolis and Saint Paul and bedroom communities grew. Today, Dakota County has a population that rivals the city of Minneapolis. Most earn their living outside the county but like many metro counties is continuing to absorb more industry and jobs from the core cities.
Dakota is named after the Dakota (or Sioux) Native Americans who were the previous predominant settlers of the area. The name is recorded Dahkotah in older U.S. Census records until 1851.
As of the census of 2000, there were 355,904 people, 131,151 households, and 94,035 families residing in the county. The population density was 625 people per square mile (241/km²). There were 133,750 housing units at an average density of 235 per square mile (91/km²). The racial makeup of the county was 91.36% White, 2.27% Black or African American, 0.38% Native American, 2.89% Asian, 0.05% Pacific Islander, 1.29% from other races, and 1.75% from two or more races. 2.94% of the population were Hispanic or Latino of any race. 32.0% were of German, 13.0% Norwegian, 9.7% Irish and 5.9% Swedish ancestry according to Census 2000. About two thirds of the county's residents live in a cluster of large suburbs in the northwestern corner of the county: Burnsville, Lakeville, Apple Valley, Rosemount and Eagan.
There were 131,151 households out of which 40.00% had children under the age of 18 living with them, 59.20% were married couples living together, 9.10% had a female householder with no husband present, and 28.30% were non-families. 21.70% of all households were made up of individuals and 5.50% had someone living alone who was 65 years of age or older. The average household size was 2.70 and the average family size was 3.19.
In the county the population was spread out with 29.20% under the age of 18, 7.90% from 18 to 24, 34.30% from 25 to 44, 21.20% from 45 to 64, and 7.40% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 97.60 males. For every 100 females age 18 and over, there were 94.70 males.
The median income for a household in the county was $61,863, and the median income for a family was $71,062 (these figures had risen to $72,393 and $86,063 respectively as of a 2007 estimate). Males had a median income of $46,827 versus $32,189 for females. The per capita income for the county was $27,008. About 2.40% of families and 3.60% of the population were below the poverty line, including 3.90% of those under age 18 and 5.50% of those age 65 or over.