According to the U.S. Census Bureau, the county has a total area of, of which is land and is water. Brasstown Bald, the highest mountain in Georgia, rises in southeast Union County, straddling the Towns County line. The central and northern portion of Union County is located in the Hiwassee River sub-basin of the Middle Tennessee-Hiwassee basin, while the southwestern portion of the county is located in the Ocoee River sub-basin of the same larger watershed. A very small portion of the county's southeastern corner is located in the Upper Chattahoochee River sub-basin of the ACF River Basin. This is also where the source of the Chattahoochee is found.
As of the census of 2000, there were 17,289 people, 7,159 households, and 5,211 families living in the county. The population density was 54 people per square mile. There were 10,001 housing units at an average density of 31 per square mile. The racial makeup of the county was 97.94% White, 0.58% Black or African American, 0.25% Native American, 0.23% Asian, 0.02% Pacific Islander, 0.24% from other races, and 0.74% from two or more races. 0.88% of the population were Hispanic or Latino of any race. Union County was mentioned as an "Extreme Whitopia", a place with a high concentration of white residents, in Rich Benjamin's book, Searching for Whitopia. There were 7,159 households out of which 24.80% had children under the age of 18 living with them, 62.90% were married couplesliving together, 7.10% had a female householder with no husband present, and 27.20% were non-families. 24.20% of all households were made up of individuals and 12.00% had someone living alone who was 65 years of age or older. The average household size was 2.35 and the average family size was 2.77. In the county, the population was spread out with 20.00% under the age of 18, 6.60% from 18 to 24, 23.60% from 25 to 44, 28.20% from 45 to 64, and 21.60% who were 65 years of age or older. The median age was 45 years. For every 100 females there were 96.60 males. For every 100 females age 18 and over, there were 94.60 males. The median income for a household in the county was $31,893, and the median income for a family was $39,776. Males had a median income of $29,127 versus $20,871 for females. The per capita income for the county was $18,845. About 9.30% of families and 12.50% of the population were below the poverty line, including 13.10% of those under age 18 and 15.90% of those age 65 or over.
2010 census
As of the 2010 United States Census, there were 21,356 people, 9,116 households, and 6,382 families living in the county. The population density was. There were 14,052 housing units at an average density of. The racial makeup of the county was 96.8% white, 0.5% black or African American, 0.4% Asian, 0.3% American Indian, 0.9% from other races, and 1.2% from two or more races. Those of Hispanic or Latino origin made up 2.4% of the population. In terms of ancestry, 16.0% were English, 15.8% were Irish, 15.0% were American, and 13.2% were German. Of the 9,116 households, 22.6% had children under the age of 18 living with them, 58.8% were married couples living together, 8.0% had a female householder with no husband present, 30.0% were non-families, and 26.2% of all households were made up of individuals. The average household size was 2.30 and the average family size was 2.75. The median age was 50.7 years. The median income for a household in the county was $41,298 and the median income for a family was $50,772. Males had a median income of $42,330 versus $29,176 for females. The per capita income for the county was $24,182. About 9.5% of families and 13.9% of the population were below the poverty line, including 14.5% of those under age 18 and 9.5% of those age 65 or over.
