Mountain Lakes was incorporated as a borough by an Act of the New Jersey Legislature on March 3, 1924, from portions of Boonton Township and Hanover Township, subject to the results of a referendum passed on April 29, 1924.
As of the census of 2000, there were 4,256 people, 1,330 households, and 1,186 families residing in the borough. The population density was 1,593.0 people per square mile (615.4/km²). There were 1,357 housing units at an average density of 507.9/sq mi (196.2/km²). The racial makeup of the borough was 93.05% White, 0.38% African American, 5.17% Asian, 0.07% Pacific Islander, 0.52% from other races, and 0.82% from two or more races. Hispanic or Latino of any race were 1.69% of the population.
There were 1,330 households out of which 53.7% had children under the age of 18 living with them, 83.3% were married couples living together, 3.8% had a female householder with no husband present, and 10.8% were non-families. 9.2% of all households were made up of individuals and 4.1% had someone living alone who was 65 years of age or older. The average household size was 3.20 and the average family size was 3.41.
In the borough the population was spread out with 35.7% under the age of 18, 3.1% from 18 to 24, 23.4% from 25 to 44, 28.7% from 45 to 64, and 9.1% who were 65 years of age or older. The median age was 39 years. For every 100 females there were 99.2 males. For every 100 females age 18 and over, there were 94.0 males.
The median income for a household in the borough was $141,757, and the median income for a family was $153,227. Males had a median income of $100,000 versus $61,098 for females. The per capita income for the borough was $65,086. About 1.4% of families and 2.0% of the population were below the poverty line, including 0.7% of those under age 18 and 2.3% of those age 65 or over.