Gold was discovered at what became the City of Yreka in 1851 which was followed by numerous other claims in Scott Valley. The formation of the California-Oregon Trail saw much of the Siskiyou Trail in the Sacramento River Canyon bypassed to the west via; Scott Valley, Scott Mountain and the town of Shasta. Nonetheless the Sacramento River Canyon and Siskiyou Trail remained an established path of travel. In 1855 a tolled bridge over the Sacramento River was built at Upper Soda Springs immediately north of modern Dunsmuir by settlers Ross and Mary McLeod. The McLeod's also constructed a sawmill and stage hotel at what is now the present site of the Mount Shasta Sisson Museum. The Siskiyou Trail would be improved by 1860 to a road capable of handing Stage Travel from Upper Soda Springs to Yreka and southward through the Sacramento River Canyon by 1870.
A small community known as "Strawberry Valley" began to develop around the Sisson Hotel and would come to be known as "Berryvale" when it was assigned Post Office Service in 1870. In 1886 local businessman Justin Sisson donated land to the Southern Pacific Railroad to construct a railroad depot on it's new line through the Sacramento River Canyon. Following the completion of the Southern Pacific Railroad line through the Sacramento River Canyon in 1887 the community of Berryvale began to grow east from the Siskiyou Trail towards it's new depot. Sisson (renamed from Berryvale in 1888) can be seen on the Southern Pacific Railroad on the 1892 Rand McNally Map of California. On May 31st, 1905 the City of Sisson incorporated and the name of "Mount Shasta" would be assumed by the Post Office on May 1st, 1924.
The emergence of the automobile in the early 20th Century in California led to the 1909 First State Highway Bond Act which was approved by voters during 1910. The majority of the highways approved as part of the First State Highway Bond Act were largely well established routes of travel. One such highway was Legislative Route Number 3 ("LRN 3") which was defined as a highway from "Sacramento to the Oregon Line."
Thusly US 99 appears on the 1925 Rand McNally Map of California as being routed through Mount Shasta City (shown as Sisson) on Mount Shasta Boulevard.
Plowing operation scenes on US 99/LRN 3 near Mount Shasta City appear on the cover of the February 1927 California Highways & Public Works.
The October 1929 California Highways & Public Works notes a contract had been awarded to pave a new 0.65 mile segment of US 99/LRN 3 north of Mount Shasta City at Spring Hill. US 99/LRN 3 was moved to a new alignment near Spring Hill to avoid a sharp curve at a steep incline. The Spring Hill realignment of US 99/LRN 3 involved relocating the Southern Pacific Railroad tracks.
The January 1930 California Highways & Public Works notes the Spring Hill realignment paving project of US 99/LRN 3 north of Mount Shasta City was opened to traffic on December 8th, 1929.
As noted in the intro the 1935 Division of Highways Map of Siskiyou County which depicts US Route 99 on Mount Shasta Boulevard.