Humboldt County is located on the far North Coast of California. In the 2000 census, the county had a population of 126,518. The two largest population centers are Eureka, the county seat, and the smaller college town of Arcata, home to Humboldt State University. Both cities are located adjacent to Humboldt Bay, California's second largest natural bay. Area cities and towns are known for hundreds of ornate examples of Victorian architecture.

According to the U.S. Census Bureau, the county has a total area of 4,052 square miles, of which, 3,572 square miles of it is land and 480 square miles of it (11.84%) is water.

Humboldt is a densely forested, rural county situated in Northern California's rugged Coast (Mountain) Ranges. With nearly 1,500,000 acres of combined public and private forest in production, Humboldt County accounts for twenty percent of the total forest production for all of California. The county contains over forty percent of all remaining old growth Coast Redwood forests, the vast majority of which is protected or strictly conserved within dozens of national, state, and local forests and parks, totaling approximately 680,000 acres (over 1,000 square miles).

The original inhabitants of the area now known as Humboldt County include the Wiyot, Yurok, Hupa, Karuk, Chilula, Whilkut, and the southern Athabascans, including the Mattole and Nongatl. Andrés de Urdaneta hit the coast near Cape Mendocino, California, then followed the coast south to Acapulco in 1565. Spanish traders made unintended visits to California with the Manila Galleons on their return trips from the Philippines beginning in 1565. Humboldt County, California was formed on 12 May 1853 from Klamath and Trinity Counties, California. The first recorded entry by people of European origin was a landing by the Spanish in 1775.

The county derived its name from Humboldt Bay. The first recorded entry of Humboldt Bay by non-natives was an 1806 visit from a sea otter hunting party from Sitka employed by the Russian American Company. The hunting party included Captain Jonathan Winship, an American, and some Aleut hunters. The bay was not visited again by people of European origin until 1849 when Josiah Gregg's party visited. In 1850, Douglas Ottinger and Hans Buhne entered the bay, naming it Humboldt in honor of the great naturalist and world explorer, Baron Alexander von Humboldt.

The area around Humboldt Bay was once solely inhabited by the Wiyot Indian tribe. One of the largest Wiyot villages, Tolowot, was located on Gunther Island (AKA Indian Island and AKA Bloody Island) in Humboldt Bay. Founded circa 900, it contains a shell midden 6 acres in size and 14 feet deep. It was the site of the February 26, 1860 massacre of the Wiyot people that was recorded by Bret Harte, then living in Union, now called Arcata. Between 60 and 200 Wiyot men, women, and children were murdered that night. In 1998, musician Frank Black wrote and recorded a song about this event, called "Humboldt County Massacre." Tolowot is now an archaeological site, designated Gunther Island Site 67, and is a National Historic Landmark.

State historic landmarks in Humboldt County include Trinidad Head, Fort Humboldt, The Old Arrow Tree, Centerville Beach Cross, Camp Curtis, the Town of Trinidad, the City of Eureka, California's first drilled oil wells in Petrolia, the Jacoby Building, the Old Indian Village of Tsurai in Trinidad, the Arcata and Mad River Railroad Company, the Humboldt Harbor Historical District, and the town of Ferndale.

On February 5 and February 6, 1885, Eureka's entire Chinese population of 300 men and 20 women were expelled after a gunfight between rival Chinese gangs (tongs) resulted in the wounding of a 12 year old boy and the death of 56 year old David Kendall, a Eureka City Councilman. After the shooting, an angry mob of 600 Eurekans met and then informed the Chinese that they were no longer wanted in Eureka and would be hanged if they were to stay in town longer than 3 p.m. the next day. They were put on two steamships and shipped to San Francisco. No-one was killed in the expulsion. Another Chinese expulsion occurred during 1906 in a cannery on the Eel River, in which 23 Chinese cannery workers were expelled after local loggers objected to their presence. However, some Chinese remained in the Orleans area, where some white landowners sheltered and purchased food for the Chinese mineworkers until after racial tension passed. Chinese did not return to the coastal cities until the 1950s.