In the 1950s construction began on the freeway portion of Route 21 and it was completed in stages between Chester Avenue in Newark and Monroe Street in Passaic between 1958 and 1973.

Plans were made to extend the freeway north to I-80 in Elmwood Park; however, they were opposed by residents living on the east side of the Passaic River.

[1][3] The route continues north, paralleling the elevated Northeast Corridor tracks that lead up to Newark Penn Station, which serves Amtrak and NJ Transit trains.

This section of Route 21 through the southern part of Newark has a high accident rate due to the heavy concentration of businesses and traffic lights along this portion of road.

[1] Route 21 and CR 508 head along the west bank of the Passaic River, passing by the New Jersey Performing Arts Center.

After passing under NJ Transit's Montclair-Boonton Line/Morris & Essex Lines and interchanging with I-280, the route intersects with Clay Street (CR 506 Spur).

It enters Nutley where the freeway interchanges with County Route 646 (Park Avenue), continuing north through residential areas along the Passaic River.

Route 21 continues to the north and resumes along the west bank of the Passaic River, narrowing to four lanes and crossing back into Clifton.

The freeway heads to the northwest, passing by a park and featuring a southbound exit and northbound entrance for Lexington Avenue before ending at an interchange with US 46.

[5][6] At that time it was set up as a surface roadway running through Newark and Belleville, with at-grade interchanges with local streets.

[13] In 1958, the highway was extended northward as a freeway along the west bank of the Passaic River to an interchange with Park Avenue in Nutley.

1970s legislation stopped the further extension northward until environmental impact could be assessed, leaving a two-mile city street portion in place to connect to Routes 20, 46, and I-80 in Paterson for over 25 years.

[16][failed verification] According to the original freeway plans, the portion north of Monroe Street was to cross over the Passaic River and terminated in Elmwood Park at the interchange of I-80 and CR 507.

In the 1980s, plans were resurrected for completing the Route 21 freeway along the west bank of the Passaic River to US 46 in Clifton, avoiding the earlier objections.