Human history in California began when indigenous Americans first arrived some 13,000 years ago. Coastal exploration by the Spanish began in the 16th century, with further European settlement along the coast and in the inland valleys following in the 18th century. California was part of New Spain until that kingdom dissolved in 1821, becoming part of Mexico.

During the Mexican–American War (1846–1848), California was ceded to the United States under the terms of the 1848 Treaty of Guadalupe Hidalgo. The same year, the California gold rush began, triggering intensified U.S. westward expansion. California joined the Union as a free state via the Compromise of 1850. By the end of the 19th century, California was still largely rural and agricultural, with a population of about 1.4 million.

The most commonly accepted model of migration to the New World is that people from Asia crossed the Bering land bridge to the Americas some 16,500 years ago. The remains of Arlington Springs Man on Santa Rosa Island are among the traces of a very early habitation, dated to the Wisconsin glaciation (the most recent ice age) about 13,000 years ago.

Some 30 tribes or culture groups lived in what is now California, gathered into perhaps six different language family groups. These groups included the early-arriving Hokan family (winding up in the mountainous far north and the Colorado River basin in the south) and the later-arriving Uto-Aztecan of the desert southeast. This cultural diversity was among the densest in North America and was likely the result of a series of migrations and invasions during the previous 13,000 years (and perhaps even earlier).

At the time of the first European contact, Native American tribes in California included the Chumash, Kumeyaay, Nisenan, Maidu, Miwok, Modoc, Mohave, Ohlone, Pomo, Serrano, Shasta, Tataviam, Tongva, Wintu, Yurok, and Yokuts. The relative strength of the tribes was dynamic, as the more successful expanded their territories and less successful tribes contracted. Slave-trading and war among tribes alternated with periods of relative peace. The total population of Native California is estimated, by the time of extensive European contact in the 18th century, to have been perhaps 300,000. Before Europeans landed in North America, about one-third of all natives in what is now the United States were living in modern-day California. California indigenous language diversity numbered 80 to 90 languages and dialects, some surviving to the present (although are endangered).

Tribes adapted to California's many climates. Coastal tribes were a major source of trading beads, which were produced from mussel shells and made using stone tools. Tribes in California's broad Central Valley and the surrounding foothills developed early agriculture, while tribes living in the mountains of the north and east relied heavily on salmon and game hunting, also collecting and shaping obsidian for themselves and trade. The harsh deserts of the southeast were home to tribes who learned to thrive by making careful use of local plants and by living in oases or along water courses. Local trade between indigenous populations enabled them to acquire seasonings such as salt or foodstuffs and other goods that might be rare in certain locales, such as flint or obsidian for making spear and arrow points.

The Native Americans had no domesticated animals except dogs and no pottery; their tools were made out of wood, leather, woven baskets and netting, stone, and antler. Some shelters were made of branches and mud; some dwellings were built by digging into the ground 2 to 3 feet (61 to 91 cm) and then constructing a brush shelter on top covered with animal skins, tules and/or mud. On the coast and somewhat inland, traditional architecture consists of rectangular redwood or cedar plank semi-subterranean houses. Traditional clothing was minimal in the summer, with tanned deerhide and other animal leathers and furs and coarse woven articles of grass clothing used in winter. Feathers were sewn into prayer pieces worn for ceremonies. Basket weaving was a high form of art and utility, as were canoe making and other carving. Some tribes around Santa Barbara and the Channel Islands were using large plank canoes to fish and trade, while tribes in the California delta and San Francisco Bay Area were using tule canoes and some tribes on the Northwest coast carved redwood dugout canoes.

A dietary staple for most indigenous populations in interior California was acorns, which were dried, shelled, ground to flour, soaked in water to leach out their tannin, and cooked. The grinding holes worn into large rocks over centuries of use are still visible in many rocks today. The ground and leached acorn flour was usually cooked into a nutritious mush, eaten daily with other traditional foods. Acorn preparation was a very labor-intensive process nearly always done by women. There are estimates that some indigenous populations might have eaten as much as one ton of acorns in one year. Families tended productive oak and tanoak groves for generations. Acorns were gathered in large quantities and could be stored as a reliable winter food source.