Hillside was incorporated as a township on April 3, 1913, from portions of Union Township, based on the results of a referendum held on April 29, 1913.
The town is split between area codes 908 and 973.
As of the census of 2000, there were 21,747 people, 7,161 households, and 5,578 families residing in the township. The population density was 7,793.6 people per square mile (3,009.5/km²). There were 7,388 housing units at an average density of 2,647.7/sq mi (1,022.4/km²). The racial makeup of the township was 40.03% White, 46.54% African American, 0.23% Native American, 3.45% Asian, 0.08% Pacific Islander, 5.26% from other races, and 4.41% from two or more races. Hispanic or Latino of any race were 14.50% of the population. 11% of the current population is of Portuguese ancestry or origin.
There were 7,161 households out of which 36.0% had children under the age of 18 living with them, 53.5% were married couples living together, 18.8% had a female householder with no husband present, and 22.1% were non-families. 18.0% of all households were made up of individuals and 7.7% had someone living alone who was 65 years of age or older. The average household size was 3.04 and the average family size was 3.45.
In the township the population was spread out with 25.6% under the age of 18, 8.9% from 18 to 24, 30.3% from 25 to 44, 24.1% from 45 to 64, and 11.1% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 88.3 males. For every 100 females age 18 and over, there were 84.2 males.
The median income for a household in the township was $59,136, and the median income for a family was $64,635. Males had a median income of $39,439 versus $31,817 for females. The per capita income for the township was $21,724. About 3.2% of families and 5.3% of the population were below the poverty line, including 5.4% of those under age 18 and 9.1% of those age 65 or over.
There is New Jersey Transit bus service to New York City and New Jersey points. There is one train line that passes through the town but there are no stations. The Irvington Branch of the Lehigh Valley Railroad breaks off of the mainline.
Newark Liberty International Airport is approximately four miles east of Hillside.
Garden State Parkway, U.S. Route 22, and Interstate 78 are located in Hillside. A toll gate is located on the northbound lanes of the parkway, approaching the interchange for 78. At the present time, the interchange between 78 & the parkway is lacking certain ramp movements due to the cancellation of Interstate 278. This is to be corrected by 2012.