According to the United States Census Bureau, the city has a total area of 3.0 square miles (7.8 km²), all of it land.
There were 3,873 households out of which 34.2% had children under the age of 18 living with them, 53.9% were married couples living together, 11.4% had a female householder with no husband present, and 28.9% were non-families. 24.7% of all households were made up of individuals and 12.0% had someone living alone who was 65 years of age or older. The average household size was 2.95 and the average family size was 3.56.
In the city the population was spread out with 25.4% under the age of 18, 9.9% from 18 to 24, 29.4% from 25 to 44, 19.9% from 45 to 64, and 15.4% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 96.0 males. For every 100 females age 18 and over, there were 92.8 males.
The median income for a household in the city was $48,406, and the median income for a family was $53,090. Males had a median income of $33,931 versus $26,313 for females. The per capita income for the city was $18,119. About 6.2% of families and 8.4% of the population were below the poverty line, including 11.3% of those under age 18 and 8.5% of those age 65 or over.
The Centerpoint Preserve (Grant Park) is a thirty two acre preserve that was donated to the City of Northlake by Centerpoint Properties. Located in the heart of Northlake, Centerpoint Preserve has become the center of activity for the city. The park has a walking/bicycle path and has created a dog park. The city also has plans for a picnic area with running water.