In the aftermath of the Mexican-American War, California existed in a state of political limbo. The peace treaty, signed in February 1848 and ratified in July, gave California to the United States—thus rendering Mexican law defunct within the ceded region’s borders. But Congress, already immersed in the heated bickering between northern and southern factional interests that would later result in outright war between the states, never got around to creating legislation that would make California an official Territory subject to American laws.
The gold discovery in late January 1848 only exacerbated an already troubling situation as—in the space of several chaotic months—tens of thousands swarmed into a previously sparsely populated region that had no system in place to make or interpret laws, or to enforce public safety.
In dire need of a civil government, prominent California residents took action themselves. In September 1849, elected delegates from ten districts convened in Monterey for a Constitutional Convention, forming committees to address the many diverse issues pertinent to the establishment of a new political jurisdiction.
Following voters’ acceptance of the new Constitution, the first session of the California Legislature met from December 15, 1849 to April 22, 1850. Based on recommendations from the convention committee tasked with the creation of counties, an Act signed on February 18, 1850, officially created 27 counties.
The original 27 were: Butte, Branciforte, Calaveras, Colusi, Contra Costa, El Dorado, Los Angeles, Marin, Mariposa, Mendocino, Monterey, Napa, Sacramento, San Diego, San Francisco, San Joaquin, San Luis Obispo, Santa Barbara, Santa Clara, Shasta, Solano, Sonoma, Sutter, Trinity, Tuolumne, Yola, and Yuba. Mariposa County, comprising about one-sixth of the terrain, was the largest by land area.
Not long afterward, subsequent statutes changed selected county names: “Branciforte” became Santa Cruz, “Colusi” became Colusa, and “Yola” became Yolo.
On September 9, 1850, California was admitted to the Union as the 31st state. At that time proposals for new counties were presented, because in just a few months’ time conflicts had already arisen between mining and agricultural districts, or over which community should be the county seat. Or—especially in some of the geographically larger counties, in an era when transportation was limited to horses and wagons—the distance citizens had to travel to reach the designated county seat. Some issues could be partly resolved by dividing counties to establish smaller new ones. Over the following years, 18 of the original 27 gave up portions of their land areas to form one or more new counties; in the 1850s decade alone, at least seven new counties were created.
Two of the proposed counties either never made it past legislative approval, or were later eliminated. Coso County was approved but never organized, and substantially the same territory became Inyo County in 1866. Klamath County, created in 1851 from the northern half of Trinity County, lost significant territory to the newly formed Del Norte County in 1857. Klamath was abolished in 1875 and its remaining territory was divided between Humboldt and Siskiyou Counties.
Imperial was the last county created, in August 1907.
Today California has 58 counties. The largest by land area is San Bernardino, with 20,057 square miles. The smallest, by land area, is San Francisco at 47 square miles. According to the 2010 Census, Los Angeles, covering 4,058 square miles, is the most populated county in California; Alpine County, containing 738 square miles, is the least populated.
More California counties are named for saints than in any other state.