He was born near Somerville in the Province of New Jersey to John Frelinghuysen (1727–1754) of Flatbush, Brooklyn and Dinah Van Berg (1725–1807) of Amsterdam. His father, John, was the son of the immigrant minister Theodorus Jacobus Frelinghuysen, the progenitor of the Frelinghuysen family in New Jersey. He graduated from the College of New Jersey (now Princeton University) in 1770, then he studied law and was admitted to the bar in 1774 and practiced law in Somerset County, New Jersey.