According to the US Census Bureau, the county has a total area of, of which is land and is water. It is the second-largest county in Montana by land area and third-largest by total area.
As of the 2000 United States Census, there were 4,601 people, 1,848 households, and 1,241 families in the county. The population density was <1/km². There were 2,502 housing units at an average density of <1/km². The racial makeup of the county was 89.44% White, 0.15% Black or African American, 7.61% Native American, 0.33% Asian, 0.02% Pacific Islander, 0.37% from other races, and 2.09% from two or more races. 1.15% of the population were Hispanic or Latino of any race. 25.3% were of German, 19.0% Norwegian, 7.6% Irish and 6.5% English ancestry. 97.0% spoke English and 2.5% German as their first language. There were 1,848 households out of which 31.90% had children under the age of 18 living with them, 57.20% were married couplesliving together, 6.80% had a female householder with no husband present, and 32.80% were non-families. 29.10% of all households were made up of individuals and 14.60% 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 3.03. The county population contained 27.30% under the age of 18, 5.50% from 18 to 24, 24.50% from 25 to 44, 25.10% from 45 to 64, and 17.60% who were 65 years of age or older. The median age was 41 years. For every 100 females there were 100.40 males. For every 100 women age 18 and over, there were 95.60 men. The median income for a household in the county was $28,702, and the median income for a family was $37,529. Males had a median income of $25,132 versus $20,274 for females. The per capita income for the county was $15,058. About 13.80% of families and 18.30% of the population were below the poverty line, including 23.10% of those under age 18 and 12.10% of those age 65 or over.
2010 census
As of the 2010 United States Census, there were 4,253 people, 1,819 households, and 1,159 families residing in the county. The population density was. There were 2,335 housing units at an average density of. The racial makeup of the county was 87.0% white, 8.3% American Indian, 0.2% Asian, 0.4% from other races, and 4.0% from two or more races. Those of Hispanic or Latino origin made up 1.9% of the population. In terms of ancestry, 26.2% were Norwegian, 24.3% were German, 13.4% were Irish, 10.1% were American, and 9.0% were English. Of the 1,819 households, 27.6% had children under the age of 18 living with them, 52.9% were married couples living together, 7.4% had a female householder with no husband present, 36.3% were non-families, and 32.2% of all households were made up of individuals. The average household size was 2.27 and the average family size was 2.86. The median age was 46.8 years. The median income for a household in the county was $36,453 and the median income for a family was $55,362. Males had a median income of $41,826 versus $26,417 for females. The per capita income for the county was $24,227. About 11.4% of families and 13.5% of the population were below the poverty line, including 17.1% of those under age 18 and 4.8% of those age 65 or over.
Politics
From its creation until 1964, voters of Phillips County were fairly balanced; they selected Democratic Party candidates in 58% of national elections. After 1964 the Republican presidential candidate has garnered the county's vote in every election.
