Adjacent counties

Major highways

National protected areas

• Charles M. Russell National Wildlife Refuge (part)
• Lewis and Clark National Forest (part)
• Upper Missouri River Breaks National Monument (part)

Military Installations

• LGM-30 Minuteman silos D-01 thru D-11 of the 10th Missile Squadron, 341st Missile Wing, 20th Air Force, assigned to Malmstrom AFB, Montana

2020 census

As of the census, of 2020, there were 11,446 people living in the county.^{[citation needed]}

2010 census

As of the 2010 census, there were 11,586 people, 5,099 households, and 3,202 families living in the county. The population density was 2.7 inhabitants per square mile (1.0/km²). There were 5,836 housing units at an average density of 1.3 per square mile (0.50/km²). The racial makeup of the county was 96.6% white, 1.2% American Indian, 0.2% black or African American, 0.2% Asian, 0.2% from other races, and 1.5% from two or more races. Those of Hispanic or Latino origin made up 1.5% of the population. In terms of ancestry, 30.8% were German, 15.9% were English, 14.7% were Norwegian, 12.8% were Irish, and 4.2% were American.

Of the 5,099 households, 23.8% had children under the age of 18 living with them, 52.9% were married couples living together, 6.5% had a female householder with no husband present, 37.2% were non-families, and 32.6% of all households were made up of individuals. The average household size was 2.18 and the average family size was 2.75. The median age was 47.8 years.

The median income for a household in the county was $37,607 and the median income for a family was $48,623. Males had a median income of $35,110 versus $21,225 for females. The per capita income for the county was $22,295. About 12.5% of families and 14.7% of the population were below the poverty line, including 24.2% of those under age 18 and 9.3% of those age 65 or over.

City

• Lewistown (county seat)

Towns

Unincorporated communities