As of the census of 2000, there were 12,366 people, 4,341 households, and 3,576 families residing in the city. The population density was 761.2 people per square mile (293.8/km²). There were 4,673 housing units at an average density of 287.6/sq mi (111.0/km²). The racial makeup of the city was 93.86% Caucasian, 3.93% African American, 0.39% Native American, 0.52% Asian, 0.01% Pacific Islander, 0.29% from other races, and 1.00% from two or more races. 1.17% of the population were Hispanic or Latino of any race.
There were 4,341 households out of which 46.0% had children under the age of 18 living with them, 67.3% were married couples living together, 11.2% had a female householder with no husband present, and 17.6% were non-families. Thirteen-point-eight percent of all households were made up of individuals and 3.3% had someone living alone who was 65 years of age or older. The average household size was 2.82 and the average family size was 3.12.
In the city the population was spread out with 30.5% under the age of 18, 6.5% from 18 to 24, 35.3% from 25 to 44, 21.4% from 45 to 64, and 6.3% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 95.4 males. For every 100 females age 18 and over, there were 93.4 males.
The median income for a household in the city was $58,600, and the median income for a family was $63,065. Males had a median income of $43,732 versus $28,515 for females. The per capita income for the city was $22,567. Two-point-four percent of the population and one-point-seven percent of families were below the poverty line. 3.2% of those under the age of 18 and 4.0% of those 65 and older were living below the poverty line.
Mt. Juliet is the largest city in Wilson County, slightly ahead of Lebanon. The city has claimed to be the "fastest-growing city in Tennessee", and it does qualify for this distinction if you consider growth from 1980–2000 for Tennessee cities with a population over 10,000. In recent years, LaVergne in Rutherford County and Spring Hill in Williamson County have grown by a larger percentage basis. Mt. Juliet remains one of the fastest growing places in Tennessee.
Interstate 40 and U.S. Route 70 (Lebanon Road) run east/west through Mt. Juliet, and State Route 171 (Mount Juliet Road) runs north-to-south connecting US-70 to I-40, before continuing toward Interstate 24 in the Antioch area.
Mt. Juliet serves as a stop on the recently-launched Music City Star commuter rail service into Nashville, operating over freight carrier Nashville and Eastern Railroad.
For commercial air traffic, Mt. Juliet is served by Nashville International Airport, located 9 miles (14 km) west of the city via Interstate 40.