Juan Rodriguez Cabrillo, Portuguese navigator exploring under a Spanish flag was in 1542 probably the first European to see California. Sir Francis Drake discovered San Francisco Bay in 1579. Father Junipero Serra founded the first of 21 missions in 1769 near San Diego. Spanish officials, however, tended to neglect California.

Russians established Fort Ross along the northern coast in 1812 as a trading and fur-trapping center. Mexico gained its independence from Spain in 1821, and the following year California became a province of the new country. During the 1840s, a few hundred U.S. citizens moved into California to live, aided by John A. Sutter, a Swiss who held a large tract of land near Sacramento. The United States went to war with Mexico in May 1846, but before that a group of U.S. citizens under the influence of John C. Fremont captured the Mexican presidio at Sonoma and raised a their own flag proclaiming the California Republic.

U.S. Navy Commodore John D. Sloat raised the U.S. flag over Monterey two months later, ending the short-lived republic. California officially was transferred to the United States on February 2, 1848, a few days after gold was discovered on the South Fork of the American River at Coloma. Throughout the next two years, the white population of the state more than tripled, reaching about 93,000. California was admitted to the Union on September 9, 1850, after heated discussions in the U.S. Congress concerning the spread of slavery. Although there was pro-slavery sentiment in southern California, the state remained in the Union during the Civil War.