It crosses the Cascade Range over White Pass, south of Mount Rainier National Park.

US 12 leaves Aberdeen and follows the Chehalis River east through Central Park as a four-lane divided highway.

[7] The highway then heads east along the Cowlitz River and passes through the town of Mossyrock, where it intersects SR 122.

[9] 12 miles (19 km) east of this intersection, US 12 crosses the Cascades over White Pass at an elevation of 4,500 feet (1,372 m).

[12] US 12 then runs concurrently with I-82, bypassing the towns of Toppenish and Prosser and paralleling the Yakima River, until exit 102 near the Tri-Cities.

Main highways in more populated areas would continue to be entirely under county control, though sometimes built with 50% state aid.

[17][18] A 1907 amendment renamed State Road 5 the Cowlitz-Natches Road, moved the Cascade crossing north to Carlton Pass, and defined the portion east of the pass to follow the Bumping River and Naches River to a point near Naches.

The majority of the route from Yakima via Connell to Pullman was not added at that time; it was finally taken over in 1937 as Secondary State Highways 11A and 11B.

Only the route via the Tri-Cities, forming part of the Inland Empire Highway, continued from Yakima to Idaho.

[30] This differed from present US 12 between Elma and Naches in that it followed the route through Olympia and Tacoma rather than along the Cowlitz River.

However, despite being part of the state highway system since 1897, US 410's crossing of the Cascades, the first between the Columbia River Gorge and Snoqualmie Pass, was not opened to traffic until 1931.

Highways were first established in 1926, US 12 ended in Miles City, Montana, and most of US 12's current routing in Washington was followed by US 410.

[43][44] Due to construction of the Mossyrock Dam on the Cowlitz River, which would create Riffe Lake and inundate parts of the valley, US 12 was relocated in December 1967 on a 16-mile (26 km) route that traveled through Morton.

[45] The detour was opened early after the Nesika Bridge over the Cowlitz River was destroyed in a fire started accidentally by construction crews a month earlier.

[50] The freeway opened on October 17, 1973, despite local opposition that compared the elevated overpasses to the Berlin Wall.

[57][58] The original opening date of May 26 was delayed by a week after issues with a paint truck needed for road striping.

[59] Construction of the final phase, bypassing Wallula Junction, remains unfunded as of 2022[update] and would require an extension of US 730 to a new interchange with US 12.