As of the census of 2000, there were 662,047 people, 263,265 households, and 175,861 families residing in the county. The population density was 595 people per square mile (230/km²). There were 288,162 housing units at an average density of 259 per square mile (100/km²). The racial makeup of the county was 58.10% White, 39.36% Black or African American, 0.21% Native American, 0.90% Asian, 0.03% Pacific Islander, 0.59% from other races, and 0.80% from two or more races. Nearly 1.55% of the population were Hispanic or Latino of any race.
There were 263,265 households, out of which 30.80% had children under the age of 18 living with them, 46.10% were married couples living together, 17.20% had a female householder with no husband present, and 33.20% were non-families. Nearly 28.70% of all households were made up of individuals, and 9.90% 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 3.04.
In the county, the population was spread out with 24.80% under the age of 18, 9.60% from 18 to 24, 29.70% from 25 to 44, 22.30% from 45 to 64, and 13.60% who were 65 years of age or older. The median age was 36 years. For every 100 females, there were 89.20 males. For every 100 females age 18 and over, there were 84.50 males.
In 2007 Jefferson County had the highest rate of syphilis cases per 100,000 in the US, according to data from the Centers for Disease Control and Prevention.
The median income for a household in the county was $36,868, and the median income for a family was $45,951. Males had a median income of $35,954 versus $26,631 for females. The per capita income for the county was $20,892. About 11.60% of families and 14.80% of the population were below the poverty line, including 20.20% of those under age 18 and 12.70% of those age 65 or over.