Marshall County lies on the north side of South Dakota. Its north boundary line abuts the south boundary line of the state of North Dakota. Its terrain consists of rolling hills, with numerous lakes and ponds in the SE portion. Its terrain slopes to the northeast, and its highest point is near its SE corner, at 2,034' ASL. Marshall County has a total area of, of which is land and is water. The county is drained by the Crow Creek, a tributary of the James River, and the Wild Rice River, a tributary of the Red River of the North. A portion of the Lake Traverse Indian Reservation is located in the eastern part of the county.
As of the 2000 United States Census, there were 4,576 people, 1,844 households, and 1,252 families in the county. The population density was 6 people per square mile. There were 2,562 housing units at an average density of 3 per square mile. The racial makeup of the county was 92.59% White, 0.09% Black or African American, 6.32% Native American, 0.09% Asian, 0.22% from other races, and 0.70% from two or more races. 0.76% of the population were Hispanic or Latino of any race. There were 1,844 households out of which 29.80% had children under the age of 18 living with them, 57.20% were married couples living together, 6.50% had a female householder with no husband present, and 32.10% were non-families. 30.10% of all households were made up of individuals and 16.10% 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 3.04. The county population contained 27.00% under the age of 18, 5.10% from 18 to 24, 22.80% from 25 to 44, 23.80% from 45 to 64, and 21.30% who were 65 years of age or older. The median age was 42 years. For every 100 females there were 100.00 males. For every 100 females age 18 and over, there were 96.80 males. The median income for a household in the county was $30,567, and the median income for a family was $36,295. Males had a median income of $27,241 versus $17,872 for females. The per capita income for the county was $15,462. About 10.40% of families and 13.90% of the population were below the poverty line, including 19.00% of those under age 18 and 14.00% of those age 65 or over.
2010 census
As of the 2010 United States Census, there were 4,656 people, 1,815 households, and 1,154 families in the county. The population density was. There were 2,534 housing units at an average density of. The racial makeup of the county was 85.4% white, 7.6% American Indian, 0.2% black or African American, 0.2% Asian, 5.6% from other races, and 1.1% from two or more races. Those of Hispanic or Latino origin made up 6.8% of the population. In terms of ancestry, 45.6% were German, 30.6% were Norwegian, 6.8% were Swedish, 5.8% were Irish, 5.2% were English, and 2.7% were American. Of the 1,815 households, 24.4% had children under the age of 18 living with them, 52.8% were married couples living together, 6.0% had a female householder with no husband present, 36.4% were non-families, and 31.4% of all households were made up of individuals. The average household size was 2.36 and the average family size was 2.83. The median age was 43.2 years. The median income for a household in the county was $41,023 and the median income for a family was $56,544. Males had a median income of $31,639 versus $26,737 for females. The per capita income for the county was $22,441. About 9.3% of families and 17.1% of the population were below the poverty line, including 27.6% of those under age 18 and 12.8% of those age 65 or over.
Marshall County voters have tended to vote Democratic for the past several decades. In 64% of the national elections since 1960, the county selected the Democratic Party candidate.
