As of the 2000 census, the city population was 5,367. A special census taken by the city in 2004 counted 7,224 residents, while the Census Bureau's estimated 2007 population for North Liberty was 10,982.
North Liberty incorporated on November 10, 1913, when its population was approximately 190. Population growth accelerated in the 1960s, and the Iowa City Community School District opened Penn Elementary there in 1961. A 1967 special census placed the city's population at 782. The population grew to over 2,000 by 1980, and in response in 1984 the North Liberty Community Library was founded and Penn Meadows Park, the city's largest, was opened. By 2005, the population of the city was over 5,000 and the city had established its own police and fire departments, and built a community center with library and aquatic park, as well as a city administration building. The city has since continued to grow, with a population of 7,224 in a 2004 special census. In 2007, the population was estimated to be 10,982, making North Liberty the second fastest growing city in Iowa.
As of the census of 2000, there were 5,367 people, 2,259 households, and 1,400 families residing in the city. The population density was 791.9 people per square mile (305.6/km²). There were 2,377 housing units at an average density of 350.7/sq mi (135.4/km²). The racial makeup of the city was 95.38% White, 1.53% African American, 0.17% Native American, 0.82% Asian, 0.11% Pacific Islander, 0.78% from other races, and 1.21% from two or more races. Hispanic or Latino of any race were 2.38% of the population.
There were 2,259 households out of which 35.4% had children under the age of 18 living with them, 48.6% were married couples living together, 9.6% had a female householder with no husband present, and 38.0% were non-families. 27.6% of all households were made up of individuals and 3.7% had someone living alone who was 65 years of age or older. The average household size was 2.38 and the average family size was 2.95.
In the city the population was spread out with 26.8% under the age of 18, 11.5% from 18 to 24, 43.9% from 25 to 44, 13.8% from 45 to 64, and 4.1% who were 65 years of age or older. The median age was 29 years. For every 100 females there were 98.0 males. For every 100 females age 18 and over, there were 96.2 males.
The median income for a household in the city was $42,500, and the median income for a family was $53,750. Males had a median income of $35,042 versus $29,328 for females. The per capita income for the city was $21,339. About 3.4% of families and 5.5% of the population were below the poverty line, including 5.4% of those under age 18 and 4.6% of those age 65 or over.