Frelinghuysen, whose father died when he was three, was the birth nephew and adoptive son of Theodore Frelinghuysen, and a member of a long-established political dynasty in New Jersey.
He was New Jersey state attorney general in 1861, US senator from New Jersey 1866-69 and 1871-77 and secretary of state under President Arthur from 1881 to 1885. He was also president of the American Bible Society in 1884-85.