Sussex was incorporated as a borough by an Act of the New Jersey Legislature on October 14, 1891 as Deckertown, from portions of Wantage. The borough was renamed as Sussex on March 2, 1902.
Sussex Borough is currently planning a consolidation study with surrounding Wantage Township. Nearby Montague Township may also take part in the study.
As of the census of 2000, there were 2,145 people, 903 households, and 512 families residing in the borough. The population density was 3,598 people per square mile (1,380/km²). There were 961 housing units at an average density of 1,612/sq mi (618/km²). The racial makeup of the borough was 96.32% White, 1.12% African American, 0.09% Native American, 1.21% Asian, 0.37% from other races, and 0.89% from two or more races. Hispanic or Latino of any race were 2.56% of the population.
There were 903 households out of which 28.3% had children under the age of 18 living with them, 38.9% were married couples living together, 12.6% had a female householder with no husband present, and 43.2% were non-families. 35.1% of all households were made up of individuals and 12.6% had someone living alone who was 65 years of age or older. The average household size was 2.36 and the average family size was 3.12.
In the borough the population was spread out with 23.9% under the age of 18, 8.5% from 18 to 24, 32.9% from 25 to 44, 22.0% from 45 to 64, and 12.7% who were 65 years of age or older. The median age was 36 years. For every 100 females there were 91.5 males. For every 100 females age 18 and over, there were 91.8 males.
The median income for a household in the borough was $36,172, and the median income for a family was $45,250. Males had a median income of $37,009 versus $22,475 for females. The per capita income for the borough was $18,866. About 6.9% of families and 11.0% of the population were below the poverty line, including 15.6% of those under age 18 and 8.4% of those age 65 or over.
Sussex is located at the intersection of Route 23 and Route 284. Sussex Airport is located one mile southwest of Sussex.