What is now Hazlet Township was originally incorporated as Raritan Township by an Act of the New Jersey Legislature on February 25, 1848, from portions of Middletown Township. Portions of the township were taken to form Holmdel Township (February 23, 1857), Matawan Township (also February 23, 1857, now Aberdeen Township), Keyport (March 17, 1870), Keansburg (March 26, 1917) and Union Beach (March 16, 1925). The township was renamed Hazlet Township as of November 28, 1967, based on the results of a referendum held on November 7, 1967. Hazlet derived its name from a Dr. John Hazlett who had an estate in Raritan Township near the Keyport-Holmdel Turnpike, now Holmdel Road.
Hazlet is part of the Bayshore Regional Strategic Plan, an effort by nine municipalities in northern Monmouth County to reinvigorate the area's economy by emphasizing the traditional downtowns, dense residential neighborhoods, maritime history, and the natural beauty of the Raritan Bayshore coastline.
As of the census of 2000, there were 21,378 people, 7,244 households, and 5,802 families residing in the township. The population density was 3,802.3 people per square mile (1,468.7/km²). There were 7,406 housing units at an average density of 1,317.2/sq mi (508.8/km²). The racial makeup of the township was 93.17% White, 1.10% African American, 0.06% Native American, 3.39% Asian, 1.13% from other races, and 1.15% from two or more races. Hispanic or Latino of any race were 5.87% of the population.
There were 7,244 households out of which 37.2% had children under the age of 18 living with them, 67.2% were married couples living together, 9.4% had a female householder with no husband present, and 19.9% were non-families. 17.3% of all households were made up of individuals and 9.7% had someone living alone who was 65 years of age or older. The average household size was 2.92 and the average family size was 3.32.
In the township the population was spread out with 25.5% under the age of 18, 6.9% from 18 to 24, 29.2% from 25 to 44, 25.0% from 45 to 64, and 13.5% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 91.5 males. For every 100 females age 18 and over, there were 89.1 males.
The median income for a household in the township was $65,697, and the median income for a family was $71,361. Males had a median income of $51,776 versus $32,439 for females. The per capita income for the township was $25,262. About 2.3% of families and 3.4% of the population were below the poverty line, including 2.6% of those under age 18 and 6.6% of those age 65 or over.
Both Route 35 and Route 36 are within Hazlet Township's borders. The nearby Garden State Parkway provides easy access to the Jersey Shore or city destinations. Mass transportation is also available via New Jersey Transit buses and by train service at the Hazlet station on the North Jersey Coast Line to Hoboken Terminal, Newark Penn Station and New York Penn Station in Midtown Manhattan. Through rail freight service is provided by Conrail Shared Assets in the form of a freight train which runs between South Amboy and Lakehurst via Red Bank.