Frederick Theodore Frelinghuysen was born on April 13, 1753, near Somerville, Somerset County, New Jersey. He was the only son of the Rev. John and Dinah (Van Berg) Frelinghuysen and the grandson of the Rev. Theodorus Jacobus Frelinghuysen who emigrated from Holland in 1720.
His father died during the son’s second year, and his mother, the daughter of a wealthy East India merchant, was about to return to her family in Amsterdam with her two small children when Jacob R. Hardenbergh a divinity student, who had been studying under her husband, persuaded her to become his wife.