Route 284 begins at a traffic light in Sussex with NJ 23. The route proceeds northward, intersecting with Sussex County Route 643 at less than a 1/2 mile. Route 284 leaves the borough of Sussex and enters Wantage Township, intersecting with local roads. Route 284 passes to the south of a lake, crosses over a branch of the Wallkill River and intersects with County Route 642. Just after the intersection with Route 642, the rural highway crosses over a small stream. At 5.41 miles, Route 284 intersects with an old routing of its original designation, NJ 84. The road merges in less than a mile later. After crossing the Wallkill River branch again, Route 284 exits New Jersey for New York.
Pre-1927 Route 8 ran largely along the present Route 23 corridor, but at Sussex it turned north rather than continuing northwest through High Point State Park. The section from Sussex north to the state line was taken over in 1919. In the 1927 renumbering, the majority of Route 8 became Route 23. The part north of Sussex was proposed to become part of Route 31, but that was instead moved to the present Route 94 corridor in the final version of the bill. As Route 8 north of Sussex was not assigned a number, the State Highway Commission appended a suffix of N (to distinguish it from new Route 8), forming Route 8N. The connecting route in New York had been numbered New York State Route 8 to match New Jersey, but in the 1930 renumbering it became New York State Route 84, in order to free up the number 8 for a longer route. New Jersey renumbered its Route 8N to Route 84 in March 1942 to match, in part to provide a single number for military caravans during World War II.
The final renumbering was made in 1966, when Interstate 84 opened in New York. As the new I-84 was close to the older Route 84, Route 84 was renumbered in both New York and New Jersey to New York State Route 284.