 Route 21 was first defined in the 1927 New Jersey state highway renumbering to run from Route 25 (now U.S. Route 1/9) and Route 29 (now U.S. Route 22) in Newark north to Belleville.The surface portion of Route 21 in Newark, which follows the Northeast Corridor rail line, was commissioned in 1934 between Routes 25 and 29 and Market Street and the portion through downtown Newark was commissioned in 1936 between Market Street and Clay Street.Route 19 was designated in 1939 from Paterson to Belleville.