Hamblen County was created in 1870 from parts of Jefferson, Grainger, and Greene counties. The county is named in honor of Hezekiah Hamblen (1775–1854), an early settler, landowner, attorney, and member of the Hawkins County Court for many years. Governor Dewitt Clinton Senter, a resident of the county, used his influence to assist in its establishment.
According to the U.S. Census Bureau, the county has a total area of 176 square miles (460 km2), of which 161 square miles (420 km2) is land and 15 square miles (39 km2) (8.3%) is water. It is the third-smallest county in Tennessee by land area and fourth-smallest by total area.
There were 24,560 households out of which 28.4% had children under the age of 18 living with them, 51.3% were married couples living together, 13.1% had a female householder with no husband present, and 30.1% were non-families. 25.70% of all households were made up of individuals living alone and 11.2% had someone living alone who was 65 years of age or older. The average household size was 2.51 and the average family size was 2.98.
In the county, the population was spread out with 23.30% under the age of 20, 5.7% from 20 to 24, 25.7% from 25 to 44, 26.7% from 45 to 64, and 15.9% who were 65 years of age or older. The median age was 39.6 years. For every 100 females there were 94.9 males. For every 100 females age 18 and over, there were 92.30 males.
The median income for a household in the county was $39,807, and the median income for a family was $48,353. Males had a median income of $36,166 versus $27,094 for females. The per capita income for the county was $21,162. 17.7% of the population and 13.2% of families were below the poverty line. Out of the total people living in poverty, 15.7% are under the age of 65 and 19.3% are 65 or older.