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 place where poplar trees grow,” A name originally given to 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, 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.