As of the census of 2000, there were 860,454 people, 352,164 households, and 213,411 families residing in the county. The population density was 2,172 people per square mile (838/km²). There were 387,183 housing units at an average density of 977 per square mile (377/km²). The racial makeup of the county was 70.49% White, 24.17% Black or African American, 0.25% Native American, 1.43% Asian, 0.04% Pacific Islander, 1.98% from other races, and 1.64% from two or more races. 3.87% of the population were Hispanic or Latino of any race. 17.0% were of German, 12.7% American, 9.0% Irish and 7.3% English ancestry according to Census 2000.
2005 Census estimates for Marion County was 65.3% non-Hispanic white, 25.8% African-American, 5.9% Latino, and 1.5% Asian
In 2000 there were 352,164 households out of which 30.20% had children under the age of 18 living with them, 41.20% were married couples living together, 14.90% had a female householder with no husband present, and 39.40% were non-families. 31.80% of all households were made up of individuals and 8.70% had someone living alone who was 65 years of age or older. The average household size was 2.39 and the average family size was 3.03.
In the county the population was spread out with 25.80% under the age of 18, 10.00% from 18 to 24, 32.90% from 25 to 44, 20.20% from 45 to 64, and 11.10% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 93.60 males. For every 100 females age 18 and over, there were 90.00 males.
The median income for a household in the county was $40,421, and the median income for a family was $49,387. Males had a median income of $36,503 versus $27,846 for females. The per capita income for the county was $21,789. About 8.70% of families and 11.40% of the population were below the poverty line, including 15.30% of those under age 18 and 8.00% of those age 65 or over.