Franklin, known as the "Fluorescent Mineral Capital of the World," is located over a rich ore body containing more than 150 minerals, many of them fluorescent and 25 of which are found no where else on earth. Settled in the 1600s, the village known as Franklin Furnace developed near iron mines and iron smelting operations located along the Wallkill River. In the early 1800s, zinc deposits in the area began to be developed commercially. For most of the century many small companies mined zinc and iron in the Franklin area. In 1897 all zinc mining efforts merged into the New Jersey Zinc Company The Zinc company was a major controlling factor in the development of Franklin. Immigrants from Russia, Britain, Hungary and Poland joined the work force at the mine. The population, 500 in 1897, had swelled to 3,000 by 1913. On March 18, 1913, the Borough of Franklin was incorporated from portions of Hardyston Township, based on the results of a referendum held on April 23, 1913.
As of the census of 2000, there were 5,160 people, 1,898 households, and 1,324 families residing in the borough. The population density was 1,150.2 people per square mile (443.7/km²). There were 1,997 housing units at an average density of 445.1/sq mi (171.7/km²). The racial makeup of the borough was 95.10% White, 0.62% African American, 0.35% Native American, 1.47% Asian, 1.22% from other races, and 1.24% from two or more races. Hispanic or Latino of any race were 4.42% of the population.
There were 1,898 households out of which 36.0% had children under the age of 18 living with them, 53.8% were married couples living together, 11.7% had a female householder with no husband present, and 30.2% were non-families. 24.1% of all households were made up of individuals and 11.3% had someone living alone who was 65 years of age or older. The average household size was 2.69 and the average family size was 3.22.
In the borough the population was spread out with 27.5% under the age of 18, 7.3% from 18 to 24, 31.8% from 25 to 44, 21.7% from 45 to 64, and 11.7% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 90.0 males. For every 100 females age 18 and over, there were 86.9 males.
The median income for a household in the borough was $44,985, and the median income for a family was $52,682. Males had a median income of $41,080 versus $26,201 for females. The per capita income for the borough was $19,386. About 5.6% of families and 7.0% of the population were below the poverty line, including 10.1% of those under age 18 and 9.9% of those age 65 or over.