From its western terminus at the Idaho state line, the UT-42 travels southeast until Cedar Creek, where it turns east-southeast. It continues in this manner until its eastern terminus at SR-30, known as Curlew Junction.
The road from SR-17 (now SR-82) in Tremonton northwest to Snowville became a state highway in 1912 as part of the Midland Trail (which continued to Nevada via present SR-30. The state highway was extended to Idaho in 1921, became part of US 30S in 1926, and received the (unsigned) SR-42 designation in 1927. The route was extended slightly east from Tremonton to SR-41 (now SR-13) in 1945, taking over what had been part of SR-82. When SR-3 was assigned to proposed I-80N in 1962, SR-42 east of the Snowville junction (exit 7) became SR-3. (Later, the portion near Tremonton would be renumbered SR-102.) Finally, in 1969, SR-42 received its present termini, as SR-70 (now SR-30) was extended east over SR-42 to I-84 west of Snowville, and the roadway through Snowville was removed from the state highway system. The American Association of State Highway Officials approved the relocation of US-30S onto I-84 in 1970, after which the first SR-42 signs were placed on the route that had been so designated since 1927.
^Utah State Legislature (1927). Chapter 21: Designation of State Roads. Session Laws of Utah. 42. From Tremonton northwesterly via Snowville to the Utah-Idaho State line near Strevell, Idaho.
^Utah State Legislature (1945). Chapter 61: State Roads and Routes. Session Laws of Utah. Route 42. From Haws Corner on SR-41 northwesterly via Tremonton and Snowville to the Utah-Idaho state line near Strevell, Idaho.