Closter, settled in 1704, was formed by an Act of the New Jersey Legislature on January 1, 1904, from portions of Harrington Township. On March 29, 1904, Harrington Park was created from portions of Closter, Harrington Township and Washington Township.
According to the United States Census Bureau, the borough has a total area of 3.3 square miles (8.5 km²), of which, 3.2 square miles (8.2 km²) of it is land and 0.1 square miles (0.3 km²) of it (3.65%) is water.
As of the 2000 census, 12.75% of Closter's residents identified themselves as being of Korean ancestry, which was the seventh highest in the United States and fifth highest of any municipality in New Jersey, for all places with 1,000 or more residents identifying their ancestry.
There were 2,789 households out of which 43.3% had children under the age of 18 living with them, 72.9% were married couples living together, 8.0% had a female householder with no husband present, and 16.8% were non-families. 14.0% of all households were made up of individuals and 7.0% had someone living alone who was 65 years of age or older. The average household size was 2.98 and the average family size was 3.30.
In the borough the population was spread out with 28.0% under the age of 18, 4.8% from 18 to 24, 28.1% from 25 to 44, 26.0% from 45 to 64, and 13.1% who were 65 years of age or older. The median age was 40 years. For every 100 females there were 97.1 males. For every 100 females age 18 and over, there were 91.0 males.
The median income for a household in the borough was $83,918, and the median income for a family was $94,543. Males had a median income of $65,848 versus $39,125 for females. The per capita income for the borough was $37,065. About 1.7% of families and 2.7% of the population were below the poverty line, including 3.0% of those under age 18 and 1.8% of those age 65 or over.
The Mayor of Closter Borough is Sophie Heymann (Republican Party, term ends December 31, 2010). Members of the Closter Borough Council are John C. Glidden, Jr. (R, 2010), Victoria Amitai (R, 2010), David Barad (R, 2008), Thomas Hennessey (R, 2009), Council President John Kashwick (R, 2008) and Cynthia L. Tutoli (R, 2009).
In elections held on November 6, 2007, voters filled two seats on the Borough Council. Incumbent Republicans Victoria R. Amitai (915 votes) and John C. Glidden, Jr. (910) were easily re-elected to second terms defeating former mayor Fred Pitofsky (650) and newcomer Raj Rane (613).
On the national level, Closter leans slightly toward the Democratic Party. In the 2004 presidential election, Democrat John Kerry received 53% of the vote here, defeating Republican George W. Bush, who received around 46%.
Public school students in grades 9-12 attend Northern Valley Regional High School at Demarest in Demarest, together with students from Demarest and Haworth. The high school is part of the Northern Valley Regional High School District, which also serves students from Harrington Park, Northvale, Norwood and Old Tappan.