The county was formed on March 25, 1853 from portions of Contra Costa and Santa Clara counties.

The word 'alameda' means 'a place where poplar trees grow', a name which originally was given to El Arroyo de la Alameda (Alameda Creek). The willow and sycamore trees along the banks of the river reminded the early explorers of a road lined with trees, also known as an 'alameda'.

The county seat at the time it was formed was located at Alvarado; it was moved to San Leandro in 1856 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 Land Grants, thence to farms, ranches, and orchards, suburbs and eventually cities, is shared with the adjacent Contra Costa County (see that article for an extensive history applicable to this county).