The county seat is Bainbridge, Georgia.
In 1825, a portion of Decatur was used in the creation of Thomas County. In 1905, another portion of Decatur was used in the creation of part of Grady County. In 1920, the western portion of Decatur County was used to form Seminole County in its entirety.
Decatur County is named for United States Navy Commodore Stephen Decatur, a hero of the War of 1812.
There were 10,380 households out of which 35.40% had children under the age of 18 living with them, 49.00% were married couples living together, 19.50% had a female householder with no husband present, and 27.30% were non-families. 24.30% of all households were made up of individuals and 10.40% had someone living alone who was 65 years of age or older. The average household size was 2.65 and the average family size was 3.14.
In the county the population was spread out with 28.50% under the age of 18, 9.10% from 18 to 24, 28.00% from 25 to 44, 21.10% from 45 to 64, and 13.30% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 91.00 males. For every 100 females age 18 and over, there were 85.90 males.
The median income for a household in the county was $28,820, and the median income for a family was $32,635. Males had a median income of $27,180 versus $20,745 for females. The per capita income for the county was $15,063. About 19.20% of families and 22.70% of the population were below the poverty line, including 33.00% of those under age 18 and 19.20% of those age 65 or over.