John graduated from Queen's College (now Rutgers University) in 1792 after studying law, and he was admitted to the bar in the state of New Jersey in 1797.

During the War of 1812, he joined the military, and commanded the New Jersey State Militia at Sandy Hook, where his assignment was to prevent ships from attacking New York City.

[1] John Frelinghuysen married Louisa Mercer in 1797, and they had two daughters: Gertrude and Mary Ann.

They had eight children: Louisa, Theodore, Frederick J., Catharine, Sallie, Sophia, and Elizabeth LaGrange Frelinghuysen.

His tombstone reads: "Sacred to the memory of General John Frelinghuysen who died on the 10th day of April A.D. 1833 in the fifty eighth year of his age ..." His c. 1801 house was donated to the Raritan Public Library in 1970 and retains its 19th-century interior finishes and exterior appearance and serves as an active community library.