Route 42 was commissioned in 1963, assembled from SR 502, SR 505 and SR 738 in Beacon Falls and Bethany. Its original extent was from Route 8 in Beacon Falls to Route 10 in Cheshire.

SR 502 was Burton Road in Beacon Falls (no longer a state road) and Beacon Road in Bethany (now part of Route 42), designated in 1962. SR 505 was Cheshire Road in Bethany, added in 1962. SR 738 was Mountain Road in Cheshire, added on Aug. 1, 1962. All these roads, except for Burton Road, became Route 42 in 1963.

The Pines Bridge Road section west of Route 8 in Beacon Falls was added in 1962 as SR 503, then renumbered to SR 703 in 1964, to fit the geographical numbering system. In 1967, Route 42 was extended over SR 703 to Route 67 in Oxford.

Improving Pines Bridge Road

In 1968, a section of Pines Bridge Road between Rimmon Hill Road and Hubbell Avenue (west of Route 8) was realigned and improved. Part of the old road exists as Old Pines Bridge Road. At the time of the public hearing, this road was still part of SR 503 and not yet part of Route 42.

Problems with the old road included: narrow (varied between 16 and 30 feet); undulating vertical profile; a low-clearance railroad underpass (9.2 feet) nicknamed "The Rabbit Hole"; and bad drainage. At the "Rabbit Hole", the road was nearly at the river's water level. The new, straighter road is 40 feet wide and passes over the railroad and Naugatuck River on a new bridge.

Plans for extending and connecting Route 42

In 1963, the Waterbury Area Transportation Study, compiled by the Naugatuck Valley Regional Planning Agency, outlined some long-term proposals for the new Route 42 to improve access in the area. The first plan was to improve Pines Bridge Road and extend it westward to Route 67. This was done by designation but not new roadway, as Route 42 zigzags toward its terminus at Route 67.

The plan also mentions Route 42 as part of a western loop of Waterbury, proceeding through Woodbury and Watertown, to connect with Route 8 at that point. That plan did not come into fruition.

In the 1970s, a few bills were submitted in the General Assembly to improve and extend Route 42.

• HB 5509, 1971: extend Route 42 to Route 34 near Stevenson Dam
• HB 5642, 1971: improve Route 42 from Route 67 to Route 10 (i.e. its entire length)
• HB 6544, 1975: improve Route 42 in Pines Bridge industrial area

Route 42 freeway: long-range proposal

In the 1960s, the South Central Connecticut Planning Region proposed a freeway in the Route 42 corridor, from Route 8 in Beacon Falls to the Wharton Brook Connector (SR 702) at I-91 exit 13, and beyond to a proposed Route 17 freeway. This seemed to fall squarely under "future needs" and apparently was not pursued with great vigor.

Addressing the mountain route

In January 1999, a bill was introduced in the General Assembly to prohibit trucks on Route 42 between Cheshire and Bethany (presumably routes 10 and 69). The road is quite narrow and sinuous through the region.

In 1999, a 90-degree curve at Back Rimmon Road in Beacon Falls (west of Route 8) was eliminated as Route 42 was realigned in the area: it now goes straight past the T-intersection and gradually curves to the north. A hump in Route 42 was also flattened to make the road safer.