As of the census of 2005, there were 123,395 people, 36,969 households, and 26,950 families residing in the city. The population density was 1,090.6 people per square mile (421.1/km²). There were 40,041 housing units at an average density of 422.1/sq mi (163.0/km²). The racial makeup of the city was 67.91% White, 23.23% African American, 0.54% Native American, 2.16% Asian, 0.25% Pacific Islander, 2.61% from other races, and 3.30% from two or more races. Hispanic or Latino of any race were 6.03% of the population. The census recorded 5,187 foreign-born residents in Clarksville.

There were 36,969 households out of which 41.3% had children under the age of 18 living with them, 56.4% were married couples living together, 13.1% had a female householder with no husband present, and 27.1% were non-families. 21.1% of all households were made up of individuals and 5.3% had someone living alone who was 65 years of age or older. The average household size was 2.69 and the average family size was 3.12.

In the city the population was spread out with 28.8% under the age of 18, 13.6% from 18 to 24, 34.7% from 25 to 44, 15.6% from 45 to 64, and 7.3% who were 65 years of age or older. The median age was 29 years. For every 100 females there were 100.9 males. For every 100 females age 18 and over, there were 98.5 males.

The median income for a household in the city was $37,548, and the median income for a family was $41,421. Males had a median income of $29,480 versus $22,549 for females. The per capita income for the city was $16,686. About 8.4% of families and 10.6% of the population were below the poverty line, including 13.8% of those under age 18 and 10.4% of those age 65 or over.

Population in recent censuses (1960-2000)

The U.S. Census says that Clarksville is the fastest-growing large city in Tennessee and the 17th in the nation. It was the only city in Tennessee to make the top 25.

The estimated population as of 2005 is 123,395 in the city overall and was on course to overtake Chattanooga as the fourth largest city in the state.

The non-profit organization FAIR projects a population of 227,300 by the year 2025, which would be a 117 percent increase from the 2000 Census, assuming that the current population increase stands.

Clarksville is a hub city for a ring of smaller, more rural counties in Tennessee and Kentucky.

Clarksville is part of the Clarksville-Hopkinsville metropolitan statistical area (MSA).