PSH 6 was originally named State Road 23 in 1915 and ran from Spokane to Newport until it was extended to the Canada–US border in 1921.

From the junction, the highway became co-signed with US 2 (formerly US 195 and US 10 Alternate and traveled northeast to intersect what was US 2 prior to 1955 in Mead.

[2][3] At Colbert, an overpass over the road served another Great Northern Railway line where the current BNSF Railway left PSH 6; from Colbert, the roadway passed Chattaroy and Milan before it left Spokane County to enter Pend Oreille County.

From Usk, the roadway continued north past Cusick, Locke, Jared and Ruby to Tiger, where SSH 6A branched off the highway and traveled west to Colville.

[3] PSH 6 bridged the Pend Oreille River at Metaline Falls and traveled north to the Canada–US border, where it became British Columbia Highway 6 (BC 6).

[19] The branch was designated in 1937 during the creation of the Primary and secondary system and became co-signed with US 195 in 1926 and US 2 in 1946,[10][12][13] the former was later shortened south to Spokane by 1979.

[22][23] From the wildlife refuge, the roadway paralleled the shoreline of Lake Thomas into Pend Oreille County and passed another mine to end at PSH 6 in Tiger.

[17][18] The busiest segment on the highway was the southern terminus near Diamond Lake, with a daily average of 600 in 1960.