The state legislature created State Route 186 in 1935. Its original route began at US-89/US-91 (now solely US-89) at the corner of State Street and 400 South, and followed the present route east to the University of Utah, but then it turned north with a gap through the university grounds, west on 200 South, and south on 1300 East to end at 2100 South, then US-40. Legislatively designated as SR-186 by the state, this was signed and treated as US-40 Alternate (US-40A) otherwise. Although SR-186 was a hidden legislative designation along the path of US-40A, the portion of 400 South between 300 West (where US-40 ran) and Redwood Road (SR-68) was solely SR-186 by 1941, and was signed as such. By 1948, the path of US-40A (hidden SR-186) ran from US-40 at the mouth of Parleys Canyon northwesterly along Foothill Drive until turning west on Sunnyside Avenue, north on 1300 East and west on 500 South and 400 South before terminating at 300 West, which then carried the designations of US-40, US-89, and US-91. By 1954, Foothill Drive was extended north past Sunnyside Avenue toward the University of Utah, and US-40A was rerouted along this new stretch of road. By 1965, US-40 and US-40A switched routes; henceforth, US-40 would run along current-day SR-186 until it was truncated in the mid-1970s. In 1966, the State Road Commission extended the route southwest on a proposed roadway to I-215, but the Bureau of Public Roads denied the request to build it with federal aid because placing an interchange at that location on I-215 was infeasible, and it was cut back to SR-68 in 1967.