Alameda County, California: History
The county was formed on March 25, 1853, from a large portion of Contra Costa County and a smaller portion of Santa Clara County.
The county seat at the time of the county's formation was located at Alvarado, now part of Union City. In 1856, it was moved to San Leandro, where the county courthouse was destroyed by the devastating 1868 quake on the Hayward Fault. The county seat was then re-established in the town of Brooklyn from 1872 to 1875. Brooklyn is now part of Oakland, which has been the county seat since 1873.
Much of what is now an intensively urban region was initially developed as a trolley car suburb of San Francisco in the late 19th and early 20th centuries. The Key System moved commuters to and from the Key System Mole, where ferries bridged the gap across San Francisco Bay.
The historical progression from Native American tribal lands to Spanish then Mexican ranches, then to farms, ranches, and orchards, then to multiple city centers and suburbs, is shared with the adjacent and closely associated Contra Costa County.