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 Spanish word alameda means "a grove of poplars.....or a tree lined street" a name originally used to describe the Arroyo de la Alameda (Poplar Grove Creek). The willow and sycamore trees along the banks of the river reminded the early explorers of a road lined with trees.
The county seat at the time it was formed 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 1872-1875. Brooklyn is now part of Oakland, which has been the county seat since 1873.
Much of what is now considered an intensively urban region, with major cities, was developed as a trolley car suburb of San Francisco in the late 19th and early 20th centuries. The historical progression from Native American tribal lands to Spanish, then Mexican ranches, then to farms, ranches, and orchards, then multiple city centers and suburbs, is shared with the adjacent and closely associated Contra Costa County.