Mount Arlington was incorporated as a borough by an Act of the New Jersey Legislature on November 3, 1890, from portions of Hanover Township, based on the results of a referendum held two days earlier. Additional territory was acquired from Roxbury Township on July 25, 1891, on May 31, 1893, and on May 31, 1894.
As of the census of 2000, there were 4,663 people, 1,918 households, and 1,262 families residing in the borough. The population density was 2,207.1 people per square mile (853.3/km²). There were 2,039 housing units at an average density of 965.1/sq mi (373.1/km²). The racial makeup of the borough was 91.42% White, 1.82% African American, 0.19% Native American, 3.82% Asian, 0.04% Pacific Islander, 1.27% from other races, and 1.44% from two or more races. Hispanic or Latino of any race were 4.55% of the population.
There were 1,918 households out of which 28.3% had children under the age of 18 living with them, 54.5% were married couples living together, 8.7% had a female householder with no husband present, and 34.2% were non-families. 27.9% of all households were made up of individuals and 6.2% had someone living alone who was 65 years of age or older. The average household size was 2.42 and the average family size was 2.99.
In the borough the population was spread out with 22.1% under the age of 18, 5.3% from 18 to 24, 35.5% from 25 to 44, 26.5% from 45 to 64, and 10.6% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 90.6 males. For every 100 females age 18 and over, there were 87.1 males.
The median income for a household in the borough was $67,213, and the median income for a family was $79,514. Males had a median income of $53,049 versus $40,417 for females. The per capita income for the borough was $32,222. About 2.3% of families and 3.3% of the population were below the poverty line, including 5.3% of those under age 18 and 3.4% of those age 65 or over.