According to the U.S. Census Bureau, the county has a total area of, of which is land and is water. It is located in the Piedmont region of the state. The western third of Lamar County, west of a line from Orchard Hill through Milner and Barnesville, is located in the Upper Flint River sub-basin of the ACF River Basin. The eastern majority of the county is located in the Upper Ocmulgee River sub-basin of the Altamaha River basin.
As of the census of 2000, there were 15,912 people, 5,712 households, and 4,284 families living in the county. The population density was 86 people per square mile. There were 6,145 housing units at an average density of 33 per square mile. The racial makeup of the county was 67.78% White, 30.39% Black or African American, 0.28% Native American, 0.37% Asian, 0.01% Pacific Islander, 0.28% from other races, and 0.89% from two or more races. 1.08% of the population were Hispanic or Latino of any race. There were 5,712 households out of which 32.20% had children under the age of 18 living with them, 54.00% were married couplesliving together, 16.30% had a female householder with no husband present, and 25.00% were non-families. 21.60% of all households were made up of individuals and 8.90% had someone living alone who was 65 years of age or older. The average household size was 2.64 and the average family size was 3.05. In the county, the population was spread out with 24.50% under the age of 18, 11.40% from 18 to 24, 27.90% from 25 to 44, 23.60% from 45 to 64, and 12.60% who were 65 years of age or older. The median age was 36 years. For every 100 females, there were 91.90 males. For every 100 females age 18 and over, there were 88.90 males. The median income for a household in the county was $37,087, and the median income for a family was $43,481. Males had a median income of $29,453 versus $21,933 for females. The per capita income for the county was $16,666. About 8.10% of families and 11.20% of the population were below the poverty line, including 14.50% of those under age 18 and 11.00% of those age 65 or over.
2010 census
As of the 2010 United States Census, there were 18,317 people, 6,618 households, and 4,617 families living in the county. The population density was. There were 7,474 housing units at an average density of. The racial makeup of the county was 66.0% white, 30.8% black or African American, 0.4% Asian, 0.3% American Indian, 0.8% from other races, and 1.7% from two or more races. Those of Hispanic or Latino origin made up 1.9% of the population. In terms of ancestry, 20.0% were American, 11.8% were English, 8.8% were Irish, and 6.0% were German. Of the 6,618 households, 31.9% had children under the age of 18 living with them, 49.0% were married couples living together, 15.7% had a female householder with no husband present, 30.2% were non-families, and 25.6% of all households were made up of individuals. The average household size was 2.55 and the average family size was 3.04. The median age was 37.7 years. The median income for a household in the county was $37,536 and the median income for a family was $42,218. Males had a median income of $37,535 versus $30,123 for females. The per capita income for the county was $17,725. About 19.4% of families and 20.4% of the population were below the poverty line, including 23.9% of those under age 18 and 17.4% of those age 65 or over.
