Originally constructed as a divided four-lane road with two roundabouts, the route of Highway 420 formed part of the QEW in 1940 initially as the southeastern terminus, but after the QEW extension from Niagara Falls to Fort Erie opened in 1941, this bypassed highway became a spur route designated as the Rainbow Bridge Approach.

It was assigned a unique route number as part of the 400-series after being upgraded into a freeway in 1972, including construction of a large interchange with the QEW.

[2] East of Montrose Road, the highway is a four lane roadway divided by a raised paved median, and passes north of a forest as it approaches a four-level interchange with the QEW.

[9][10] Now separated by a landscaped median, the route progresses eastward, intersecting MacDonald Avenue and providing access to several residential and retail properties that adjoin the road.

The route travels within a concrete trench and abruptly curves to the southeast as it passes beneath Victoria Avenue, with which there is a simple interchange.

At the bottom, the route curves to the southwest, where it provides access to the Rainbow Bridge border crossing into the United States as well as the tourist district of the city.

Its narrow design proved to be a fatal flaw, and on January 27, 1938, under the weight of a massive ice jam in the river, the structure collapsed.

[13] A day later, the owners of the previous bridge—the International Railway Company (IRC)—as well as Ontario Minister of Highways Thomas Baker McQuesten announced intentions to construct a new span; a long political battle ensued for several years over the merits of private or public ownership of border crossings.

[15] On May 16, 1940, Samuel Johnson, the vice-chair of the NFBC, and McQuesten, who along with his parliamentary role was chair of the commission, ceremonially turned the first sod for the new bridge using a two-handled shovel.

[16] At the same time construction on QEW between Niagara Falls and Fort Erie, was underway, but the ongoing war delayed its completion.

[22] During the mid-1960s, the Department of Highways examined the possibility of extending the freeway portion of the route east towards the Rainbow Bridge.

[25][26] In 1998, the Niagara Falls Transportation Study was released, recommending that Roberts Street be rebuilt as a gateway to the city as opposed to a freeway.

[23] In the early 2000s, Highway 420 from the QEW interchange to Drummond Road was reconstructed with high-mast lighting poles and an Ontario tall-wall concrete median Jersey barrier, similar to the upgrades that other high-traffic provincial freeways received around that time.

The Drummond Road and Portage Road overpasses that originally built in 1941 and rehabilitated in the early 1970s were demolished in 2004; their new replacement bridges plus Highway 420's concrete median barrier feature decorative stone facades, which in combination with the "ER" light poles restores the heritage look that was lost in the 1970s reconstruction.

[28] On January 31, 2012, Niagara Regional Council approved the renaming of Roberts Street and Newman Hill as an extension of Falls Avenue, beginning March 1.