According to the U.S. Census Bureau, the county has a total area of, of which is land and is water. The majority of Miller County, west of a north-to-south line made as a continuation of the eastern Early County border, is located in the Spring Creek sub-basin of the ACF River Basin. The county's very northeastern corner is located in the Ichawaynochaway Creek sub-basin of the same ACF River Basin, while the southeastern portion, from just north of State Route 91going south, is located in the Lower Flint River sub-basin of the same larger ACF River Basin.
As of the census of 2000, there were 6,383 people, 2,487 households, and 1,765 families living in the county. The population density was 9/km². There were 2,770 housing units at an average density of 4/km². The racial makeup of the county was 70.26% White, 28.90% Black or African American, 0.17% Native American, 0.05% Asian, 0.08% Pacific Islander, 0.20% from other races, and 0.33% from two or more races. 0.69% of the population were Hispanic or Latino of any race. There were 2,487 households out of which 31.50% had children under the age of 18 living with them, 50.70% were married couplesliving together, 15.50% had a female householder with no husband present, and 29.00% were non-families. 26.70% of all households were made up of individuals and 13.60% had someone living alone who was 65 years of age or older. The average household size was 2.51 and the average family size was 3.03. In the county, the population was spread out with 26.30% under the age of 18, 7.00% from 18 to 24, 26.20% from 25 to 44, 23.40% from 45 to 64, and 17.10% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 89.10 males. For every 100 females age 18 and over, there were 83.90 males. The median income for a household in the county was $27,335, and the median income for a family was $31,866. Males had a median income of $25,995 versus $20,886 for females. The per capita income for the county was $15,435. About 16.90% of families and 21.20% of the population were below the poverty line, including 28.70% of those under age 18 and 21.10% of those age 65 or over.
2010 census
As of the 2010 United States Census, there were 6,125 people, 2,426 households, and 1,674 families living in the county. The population density was. There were 2,791 housing units at an average density of. The racial makeup of the county was 69.6% white, 28.1% black or African American, 0.5% Asian, 0.2% American Indian, 0.4% from other races, and 1.2% from two or more races. Those of Hispanic or Latino origin made up 1.5% of the population. In terms of ancestry, 14.2% were American, and 5.2% were Irish. Of the 2,426 households, 32.2% had children under the age of 18 living with them, 49.1% were married couples living together, 15.6% had a female householder with no husband present, 31.0% were non-families, and 27.3% of all households were made up of individuals. The average household size was 2.46 and the average family size was 3.00. The median age was 41.7 years. The median income for a household in the county was $33,196 and the median income for a family was $40,685. Males had a median income of $31,985 versus $29,110 for females. The per capita income for the county was $19,895. About 18.9% of families and 18.2% of the population were below the poverty line, including 22.3% of those under age 18 and 16.1% of those age 65 or over.
