Morris Plains was incorporated as a borough by an Act of the New Jersey Legislature on March 15, 1926, from portions of Hanover Township, based on the results of a referendum held on April 15, 1926.
Morris Plains serves as a bedroom community for workers in New York City. The Morristown Line of New Jersey Transit provides commuters with direct access to New York Penn Station and to Hoboken Terminal.
Morris Plains is also a major base of operations for Pfizer, previously the headquarters for Warner-Lambert. Pfizer also sponsors the Health and Medical Science Academy at Morristown High School and has an annual 5k race around Morris Plains.
As of the census of 2000, there were 5,236 people, 1,955 households, and 1,477 families residing in the borough. The population density was 2,018.7 people per square mile (780.6/km²). There were 1,994 housing units at an average density of 768.8/sq mi (297.3/km²). The racial makeup of the borough was 92.91% White, 1.34% African American, 0.06% Native American, 4.32% Asian, 0.10% Pacific Islander, 0.40% from other races, and 0.88% from two or more races. Hispanic or Latino of any race were 2.69% of the population.
There were 1,955 households out of which 33.1% had children under the age of 18 living with them, 67.0% were married couples living together, 7.0% had a female householder with no husband present, and 24.4% were non-families. 19.7% of all households were made up of individuals and 8.6% had someone living alone who was 65 years of age or older. The average household size was 2.63 and the average family size was 3.05.
In the borough the population was spread out with 23.5% under the age of 18, 4.4% from 18 to 24, 29.4% from 25 to 44, 26.4% from 45 to 64, and 16.2% who are 65 years of age or older. The average age is 41 years. For every 100 females there were 92.2 males. For every 100 females age 18 and over, there were 86.6 males.
The median income for a household in the borough was $84,806, and the median income for a family was $98,333. Males had a median income of $75,040 versus $44,554 for females. The per capita income for the borough was $36,553. About 1.5% of families and 2.4% of the population were below the poverty line, including 3.0% of those under age 18 and 2.4% of those age 65 or over.