The County of San Diego was established by the State Legislature on February 18, 1850, as one of the original 27 counties of California.

A Court of Sessions was created that same year to handle administrative affairs of County government, but was replaced in 1852 by a five-member Board of Supervisors, also created by the Legislature. The San Diego County Board of Supervisors held its first meeting on January 3, 1853 when the County was divided into supervisorial districts.

At the time of its creation, San Diego County had an estimated population of at least 3490. According to historians, this estimate included 798 settlers, as well as approximately 2692 Native Americans in the County. The newly created county covered nearly 40,000 square miles. Included in the original county boundary were the present counties of San Diego, Imperial, Riverside, San Bernardino, and the eastern portion of Inyo County. Today, San Diego County covers approximately 4,255 square miles - 65 miles from north to south and 86 miles from east to west.

The County draws its name from San Diego de Alcala, a designation credited to Spaniard Don Sebastian Vizcaino who sailed into what is now San Diego Bay on November 12, 1603, and renamed it in honor of his flagship and, it is said, his favorite saint. The site was actually discovered 61 years earlier by Juan Rodriguez Cabrillo who had named it San Miguel.