Route 18-N was defined in 1923 to run "from Hoboken to New York State line by way of Weehawken, West Hoboken, town of Union, North Bergen, Fairview, Ridgefield, Palisade Park, Fort Lee, Englewood-Cliffs, Tenafly and Alpine."

Route 18-N continued along the current-day abandoned alignment of U.S. Route 9W, where the highway passed along the cliffs of the Palisades until crossing the state line into New York near Alpine.

[1] State Highway Route 18-N was first designated in the 1923 Annual Report on Public Roads as running from the city of Hoboken northward through several communities in Bergen and Hudson County to the New York state line at Alpine.

[2] The portion from Hoboken to Fort Lee was never officially taken over by the New Jersey State Highway Department, as when the 1927 state highway renumbering occurred, the alignment of Route 18-N south of Fort Lee had already wiped,[3] but the designation was retained from the Fort Lee to the state line in Alpine.

[5] This former alignment of Route 18-N through the Palisades Cliffs has since been abandoned and was part of the Long Path, a hiking trail and scenic overlook as Old Route 9W until the Long Path was realigned on April 1, 2009.