John Frelinghuysen married Louisa Mercer in 1797, and they had two daughters: Gertrude and Mary Ann. Louisa died around 1809 and John married Elizabeth Mercereau Van Vechten on November 13, 1811. They had eight children: Louisa, Theodore, Frederick J., Catharine, Sallie, Sophia, and Elizabeth LaGrange Frelinghuysen.
John Frelinghuysen died in Millstone, New Jersey on April 10, 1833, and was buried in the Old Somerville Cemetery in Somerville, New Jersey. 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 ..."
Legacy
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.[2]