Mendocino County was one of California's original 27 counties, created in 1850 by the State Legislature. Because of its small population, Mendocino County was administered by the government of Sonoma County until 1859, when the government was established in a small building on Main Street in Ukiah. County officials moved into the first courthouse at the site bounded by Standley, Perkins, State and Schools streets on January 24, 1860.

Mendocino County derives its name from Cape Mendocino, which lies northward of its northern boundary. Cape Mendocino was given its name by a famous Spanish navigator of the 16^th century, Juan Rodriquez Cabrillo. Cabrillo discovered it in 1542 while on a voyage of discovery along the Pacific Coast and named it in honor of Don Antonio de Mendoza, the first Viceroy of New Spain (Mexico), and the patron of the voyageur.

The first permanent Spanish settlers came to the area in the middle 16^th century. It was almost 300 years before the first permanent non-Spanish settlements in the county were made on the coast north of Big River in April of 1852.

Mendocino County encompasses an area of over 2 million acres or approximately 3500 square miles.