Cayoosh Pass and the valleys of Cayoosh and Joffre Creeks form the southern boundary of the Cayoosh Range, a subrange of the Lillooet Ranges. Long known to the St'at'imc and Lil'wat peoples whose territories include it, the pass was first traversed by a non-indigenous person when James Duffey, a.k.a. Sapper Duffy of the Royal Engineers, investigated the route in 1859-1860 during a resurvey and reconstruction of the Douglas Road, the route of which passed the Joffre Creek foot of the pass and followed the northern perimeter of the Cayoosh Range. Cayoosh Pass was reported by Sapper Duffey to be too steep for wagons and any thought of a road by that route was shelved until the later 20th Century. Newer engineering techniques in the 1970s saw a surge in logging road construction in the Pemberton area, which came over the summit of the pass into valleys south of Duffey Lake. Logging roads from the Lillooet side eventually linked up with the Pemberton-side roads by the late 1970s and this route was ultimately chosen for the extension of Highway 99 northwards from Pemberton, over the other available routes were one via Railroad Pass and the Hurley River, to the north of Pemberton, and another via Anderson and Seton Lakes, the route followed by the railway.