The county was created from the east half of Norman County on December 27, 1906, with Mahnomen, a former railway station town, as the county seat. The county was named for the town, the name of which is one spelling of the Ojibwe word for "wild rice".
Geography
The Wild Rice River enters the county from Clearwater County and flows west through the central part of the county. The White Earth River originates from White Earth Lake on the county's southern border and flows northwest to its confluence with the Wild Rice near Mahnomen. The county terrain consists of low rolling hills, carved with drainages. The eastern part of the county is dotted with lakes and ponds and largely wooded. All non-wooded areas are devoted to agriculture where possible. The county slopes to the west and north, with its highest point near the middle of the east border, at 1,768' ASL. The county has an area of, of which is land and is water. Mahnomen is one of 17 Minnesota savanna region counties with more savanna soils than either prairie or forest soils.
As of the 2000 United States Census, there were 5,190 people, 1,969 households, and 1,366 families in the county. The population density was 9.3/sqmi. There were 2,700 housing units at an average density of 4.84/sqmi. The racial makeup of the county was 62.85% White 0.13% Black or African American, 28.55% Native American, 0.06% Asian, 0.31% from other races, and 8.09% from two or more races. 0.89% of the population were Hispanic or Latino of any race. 29.4% were of German and 17.0% Norwegian ancestry. There were 1,969 households out of which 32.40% had children under the age of 18 living with them, 51.60% were married couples living together, 11.60% had a female householder with no husband present, and 30.60% were non-families. 27.00% of all households were made up of individuals and 14.90% had someone living alone who was 65 years of age or older. The average household size was 2.60 and the average family size was 3.14. The county population contained 29.20% under the age of 18, 7.20% from 18 to 24, 23.50% from 25 to 44, 23.40% from 45 to 64, and 16.70% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 102.90 males. For every 100 females age 18 and over, there were 98.40 males. The median income for a household in the county was $30,053, and the median income for a family was $35,500. Males had a median income of $23,614 versus $21,000 for females. The per capita income for the county was $13,438. About 11.80% of families and 16.70% of the population were below the poverty line, including 21.30% of those under age 18 and 15.30% of those age 65 or over.
For several decades, Mahnomen County voters have tended to vote Democratic. As of 2016 the county has selected the Democratic candidate in six of the nine presidential elections since 1980.
