The highway travels across Whidbey Island, North Cascades National Park, the Okanagan Highland, the Kettle River Range, and the Selkirk Mountains.

SR 20's path across the Cascades follows one of the oldest state roads in Washington, established in 1896 as a wagon route.

The development of the Skagit River Hydroelectric Project spurred boosters to lobby the state for a new route across Washington Pass, which was opened in 1972.

The highway travels northerly through the rural Quimper Peninsula and follows the east shore of Discovery Bay before reaching a junction with SR 19 near Jefferson County International Airport.

Within Port Townsend, the highway travels east through suburban areas as Sims Way and towards the historic downtown as Water Street.

To the southwest of Port Townsend's historic district, the route turns southeast into the city ferry terminal.

SR 20 continues onto the Port Townsend–Coupeville ferry which travels northeast across the Admiralty Inlet to the Keystone terminal on Whidbey Island.

[6] From the Keystone ferry terminal, located adjacent to Fort Casey State Park, the highway travels east between the shore of Admiralty Bay and Crockett Lake, while making a short diversion to the north.

The highway, designated as part of the Cascade Loop and Whidbey Scenic Isle Way state scenic byways, passes the Naval Outlying Landing Field Coupeville and Rhododendron County Park before taking a turn west towards Coupeville and Ebey's Landing National Historical Reserve.

[7] The highway continues northeast into Oak Harbor, where it travels around the downtown area and heads towards Naval Air Station Whidbey Island.

[5] SR 20 continues northeast, passing between Lake Campbell and Similk Bay, to Sharpes Corner, where it is joined by a spur route serving Anacortes.

The highway follows a branch of the BNSF Railway, serving the Shell and Marathon oil refineries on March's Point, along the south side of Padilla Bay.

Near Skagit Regional Airport, it intersects SR 536, which connects to Mount Vernon, and veers northeast along a four-lane expressway.

The highway crosses Rainy Pass and briefly joins the Pacific Crest Trail as it travels around Whistler Mountain, climbing up into Chelan County.

[5][9] SR 20 travels northeast along Early Winters Creek and the base of Delancy Ridge towards Mazama, where it meets the Methow River.

The highway travels north through the farming community of Riverside to Tonasket, where SR 20 leaves US 97 and takes a turn east towards the Okanogan Highlands.

The highway dips south into the Colville National Forest and climbs the ridge, crossing Sherman Pass in a series of hairpin turns.

[11] SR 20 descends along Sherman Creek towards Franklin D. Roosevelt Lake on the Columbia River, which it crosses on a highway concurrent with US 395.

[14] What is known today as the North Cascades Highway was originally the corridor used by local Native American tribes as a trading route from Washington's Eastern Plateau country to the Pacific Coast for more than 8,000 years.

Funds were also allotted to improve access roads on both sides of the North Cascades and construction on this section of the highway began in 1959.

[15] Over the next nine years, construction of the road continued along with the signing of the North Cascades National Park bill by President Lyndon B. Johnson in 1968.

Nonetheless, businessmen and residents on both sides of the North Cascades were hopeful and supportive of the tourist dollars that would be seen with the opening of the "North-Cross Highway".

[16] The North Cascades Highway officially opened on September 2, 1972, with a procession led over Washington Pass by governor Dan Evans and President Richard M. Nixon's brother Edward.

[17][18] The highway's construction came with conservation measures and upgrades to camping facilities in the then-new North Cascades National Park to handle the expected traffic.

The route turns northward at a round about, one block west of R Ave, to become Commercial Avenue heading toward downtown Anacortes.