Its west end is at SR 1 in Fort Bragg, from where it heads east past Clear Lake, Colusa, Yuba City, Marysville and Nevada City to I-80 near Emigrant Gap, where eastbound traffic can continue on other routes to Lake Tahoe or Nevada.

State Route 20 begins at SR 1 in southern Fort Bragg, less than 1 mile (1.6 km) from the Pacific Ocean.

The highway continues to rise alongside the North Fork Big River and tributaries, crossing another summit and then descending to Willits in the Little Lake Valley via Broaddus Creek.

SR 20 closely follows the northeast shore of Clear Lake, staying right above the water line to avoid the adjacent hills.

The highway leaves Marysville to the northeast, paralleling the Yuba River on its north side as it enters the foothills of the Sierra Nevada.

[14] By the end of the 1910s, a passable dirt and gravel road connected Ukiah and Nevada City via the south side of Clear Lake and Marysville.

This route generally followed the present SR 20, except around Clear Lake and between Marysville and Rough and Ready (where it used Spenceville Road).

[18] The law that defined the extension simply stated that it would connect "Ukiah to Tahoe City";[19] the state decided in September 1925 that it would run the highway along the north shore of Clear Lake, combining with the planned Rumsey-Lower Lake Highway (Route 50, now SR 53 and SR 16) east to Wilbur Springs.

[23] In 1953, the legislature added an extension of Route 15 from US 101 at Willits (north of Ukiah) west to SR 1 near Fort Bragg.

[28] Except where prefixed with a letter, postmiles were measured on the road as it was in 1964, based on the alignment that existed at the time, and do not necessarily reflect current mileage.