By 1916, a road between Napavine and Cinebar was constructed on the current route of SR 508 and was signed in 1937 as Secondary State Highway 5K (SSH 5K) after being extended to Morton.

East of Cinebar, the roadway starts to parallel the Tilton River through Bear Canyon to Morton, where the road becomes Main Avenue and ends at SR 7, named Second Street.

[6] A large flood event in December 1977, due to heavy rainfall and snow melt, severely impacted eastern Lewis County.

A log jam led to the destruction of the route's 150 foot (46 metres)-tall Bear Canyon Bridge when the span's sole support tower was broken, fully collapsing the overpass.

In early November 2006, heavy rainfall at Bear Canyon[18] resulted in erosion of a cliff on the highway that caused portions of the roadway to fall into the Tilton River.