- Total 4,526 sq mi (11,720 km2)
- Land 4,207 sq mi (10,900 km2)
- Water 319 sq mi (830 km2)
Highest elevation 6,536 ft (1,992 m)
Population (April 1, 2010)
- Total 3,095,313
- Estimate (2017) 3,337,685
- Density 680/sq mi (260/km2)
The area which is now San Diego County has been inhabited for more than 12,000 years by Kumeyaay (also called Diegueno and Ipai/Tipai), Luiseño, Cupeño and Cahuilla Indians and their local predecessors.
In 1542, the explorer Juan Rodríguez Cabrillo, who may have been born in Portugal but sailed on behalf of Spain, claimed San Diego Bay for the Spanish Empire, and he named the site San Miguel. In November 1602, Sebastián Vizcaíno surveyed the harbor and what are now Mission Bay and Point Loma and named the area for Saint Didacus, a Spaniard more commonly known as San Diego. European settlement in what is now San Diego County began with the founding of the San Diego Presidio and Mission San Diego de Alcalá by Spanish soldiers and clerics in 1769. This county was part of Alta California under the Viceroyalty of New Spain until the Mexican declaration of independence. From 1821 through 1848 this area was part of Mexico.
San Diego County became part of the United States as a result of the Treaty of Guadalupe Hidalgo in 1848, ending the Mexican–American War. This treaty designated the new border as terminating at a point on the Pacific Ocean coast which would result in the border passing one Spanish league south of the southernmost portion of San Diego Bay, thus ensuring that the United States received all of this natural harbor.
San Diego County was one of the original counties of California, created at the time of California statehood in 1850.:221
At the time of its establishment in 1850, San Diego County was relatively large, and included all of southernmost California south and east of Los Angeles County. It included areas of what are now Inyo and San Bernardino Counties, as well as all of what are now Riverside and Imperial Counties.:221
During the later part of the 19th century, there were numerous changes in the boundaries of San Diego County, when various areas were separated to make up the counties mentioned above. The most recent changes were the establishments of Riverside County in 1893:207 and Imperial County in 1907.:113 Imperial County was also the last county to be established in California, and after this division, San Diego no longer extended from the Pacific Ocean to the Colorado River, and it no longer covered the entire border between California and Mexico.