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.
According to the United States Census Bureau, the borough has a total area of 2.9 square miles (7.5 km²), of which, 2.7 square miles (6.9 km²) of it is land and 0.2 square miles (0.6 km²) of it (7.61%) is water.
Part of The Tourne county park is in Mountain Lakes.
The man-made lakes in Mountain Lakes are: Birchwood Lake, Crystal Lake, Mountain Lake, Sunset Lake, Wildwood Lake, and Cove Lake. It is only legal to swim in Birchwood Lake, and Mountain Lake in the areas that are roped off. You may swim between the hours of 5 am to 10 Pm every day between the months of June and August.
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.
Members of the Mountain Lakes Borough Council are Mayor George Jackson, Deputy Mayor Louise Davis, Ellen Emr, Charles Gormally, Blair Schleicher-Wilson, Stephen Shaw and Richard Urankar.
Mountain Lakes is also home to The Craig School, a private coeducational day school serving students in third through twelfth grade. The school has an enrollment of 160 students split between the Lower School (grades 3-8), located in Mountain Lakes, and the Upper School (grades 9-12), in Lincoln Park.