Route 42 begins at an intersection with Route 67 just east of the town center of Oxford as Chestnut Tree Hill Road Extension .

0.7 miles (1.1 km) beyond the Route 8 junction, Route 42 turns east onto Bethany Road while South Main Street continues north into Beacon Falls center.

About two miles (3 km) later, the road ends at Old Litchfield Turnpike near the junction with Route 63.

In Cheshire, the road name becomes Bethany Mountain Road as it traverses the ridge separating Prospect/Bethany and Cheshire (part of the Metacomet Ridge).

In 1968, Route 42 was relocated southward to bypass Beacon Falls center to its modern alignment along Bethany Road.