The state legislature defined State Route 73 in 1933, running from SR-68 (created in 1931) west of Lehi southwest via Cedar Fort, Fairfield and Topliff to SR-36. Up until Fivemile Pass, beyond Fairfield, this followed the present route of SR-73, but at Fivemile Pass the route curved southwesterly, generally following the former Fairfield Branch of the Union Pacific Railroad. Two years later, the portion west of Fairfield was dropped, in favor of a routing south from Fairfield to SR-26 (now US-6) near Eureka. State Route 180 was also created in 1935, beginning at SR-36 south of Stockton and heading east through Ophir Canyon to Ophir and southeast through the Oquirrh Mountains to Mercur. State Route 202 was defined in 1939 as an eastern approach to Mercur, splitting off SR-73 north of Fairfield and climbing Manning Canyon, and in 1941 the portion of SR-180 between Ophir and Mercur was removed from the state highway system, with an extension of SR-202 west down Mercur Canyon to SR-180 replacing it.