Alameda County ( ) is located in the U.S. state of California. As of the 2020 census, the population was 1,682,353, making it the 7th-most populous county in the state and 21st most populous nationally. The county seat is Oakland. Alameda County is in the San Francisco Bay Area, occupying much of the East Bay region.
The Spanish word alameda means either "a grove of poplars...or a tree lined street". The name was originally used to describe the Arroyo de la Alameda; the willow and sycamore trees along the banks of the river reminded the early Spanish explorers of a road lined with trees.
Alameda County is part of the San Francisco–Oakland–Berkeley, CA Metropolitan Statistical Area, and the San Jose–San Francisco–Oakland, CA Combined Statistical Area.
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.
Law, government and politics
thumb|[[René C. Davidson Courthouse, Alameda County Superior Court, Oakland, in June 2009]]
Government
The Government of Alameda County is defined and authorized under the California Constitution, California law, and the Charter of the County of Alameda. Much of the Government of California is in practice the responsibility of county governments such as the Government of Alameda County, while municipalities such as the City of Oakland and the City of Berkeley provide additional, often non-essential services. The County government provides countywide services such as elections and voter registration, law enforcement, jails, vital records, property records, tax collection, and public health. In addition it is the local government for all unincorporated areas, and provides services such as law enforcement to some incorporated cities under a contract arrangement.
It is composed of the elected five-member Alameda County Board of Supervisors (BOS) as the county legislature, several other elected offices and officers including the Sheriff, the District Attorney, Assessor, Auditor-Controller/County Clerk/Recorder, Treasurer/Tax Collector, and numerous county departments and entities under the supervision of the County Administrator. In addition, several entities of the government of California have jurisdiction conterminous with Alameda County, such as the Alameda County Superior Court.
As of 2026, the supervisors are:
- David Haubert (President), district 1,
- Elisa Márquez, district 2,
- Lena Tam (Vice President), district 3,
- Nate Miley, district 4, and
- Nikki Fortunato Bas, district 5.
The Board elects a president who presides at all meetings of the Board and appoints committees to handle work involving the major programs of the county. If the president is absent for a meeting, the vice president shall be responsible. A Board election occurs every two years for these positions. Supervisor Carson is serving currently as president; Supervisor Miley is vice president.
The county's law enforcement is overseen by an elected Sheriff/Coroner and an elected District Attorney. The Sheriff supervises the deputies of the Alameda County Sheriff's Office, whose primary responsibilities include policing unincorporated areas of the county and cities within the county which contract with the Sheriff's Office for police services; providing security and law enforcement for county buildings including courthouses, the county jail and other county properties; providing support resources, such as a forensics laboratory and search and rescue capabilities, to other law enforcement agencies throughout the county; and serving the process of the county's Superior Court system. The District Attorney's office is responsible for prosecuting all criminal violations of the laws of the state of California, the county, or its constituent municipalities, in the Alameda County Superior Court. The current Sheriff is Yesenia Sanchez, who was elected in 2022, succeeding Greg Ahern, who had served in the post for 16 years. The Sheriff's Office operates two jails: Santa Rita Jail in Dublin, and Glenn E. Dyer Detention Facility in downtown Oakland.
In 2009, Nancy E. O'Malley was appointed Alameda County district attorney after Tom Orloff retired. She served two terms and did not run for reelection in 2022. Pamela Price was elected as district attorney in 2022.
The Alameda County Fire Department (ACFD) was formed on July 1, 1993, as a dependent district, with the Board of Supervisors as its governing body. Municipal and specialized fire departments have been consolidated into the ACFD over the years. 1993 brought in the Castro Valley and Eden Consolidated FD, and the County Fire Patrol. San Leandro joined in 1995, Dublin in 1997, Lawrence Berkeley National Laboratory in 2002, Lawrence Livermore National Laboratory in 2007, The Alameda County Regional Emergency Communications Center in 2008, and Newark and Union City in 2010. Emeryville joined the ACFD in 2012.
The Alameda County Water District is a special district within Alameda County created to distribute water, but it is not operated by Alameda County administrators. It is operated by an elected board of directors.
Alameda County Superior Court operates in twelve separate locations throughout the county, with its central René C. Davidson Courthouse located in Oakland near Lake Merritt. Most major criminal trials and complex civil cases are heard at this location or in courtrooms within the County Administration Building across the street.
State and federal representation
In the California State Assembly, Alameda County is split between five districts:
In the California State Senate, the county is split between four districts:
In the United States House of Representatives, the county is split between four districts:
Politics
Since 1932, Alameda County has been a stronghold of the Democratic Party, with Dwight Eisenhower being the only Republican presidential nominee to have carried the county since. Prior to 1932, the county had been a Republican stronghold. Piedmont resident William F. Knowland was the Republican U.S. Senate Leader from 1953 to 1959. Even when Ronald Reagan won the national popular vote by an 18.3% margin in 1984, Walter Mondale won Alameda County by a larger margin. In 2004 it voted for John Kerry, who won over 75% of the vote. Every city and town voted Democratic. George H.W. Bush in 1988 was the last Republican to break 30% of the county's vote and Ronald Reagan in 1984 was the last to break 40% of the vote (carrying 40.01%).
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On November 4, 2008, Alameda County voted 61.92% against Proposition 8, which won statewide, and which amended the California Constitution to ban same-sex marriage. The county garnered the sixth highest "no" vote, by percentage, of all California counties, and was the second largest county, by total voter turnout, to vote against it.
Voter registration statistics as of October 24, 2022
{| class="wikitable collapsible collapsed"
|-
! colspan="3" | Population and registered voters
|-
! scope="row" style="text-align: left;" | Total eligible population
| colspan="2" | 1,140,774
|-
! scope="row" style="text-align: left;" | Registered voters
| 931,130
| 81.6%
|-
! scope="row" style="text-align: left;" | Democratic
! data-sort-type="number" | Registered voters<br /> The San Francisco Bay borders the county on the west.
The crest of the Berkeley Hills forms part of the northeastern boundary and reaches into the center of the county. A coastal plain several miles wide lines the bay; and is Oakland's most populous region. Livermore Valley lies in the eastern part of the county. Amador Valley abuts the western edge of Livermore Valley and continues west to the Pleasanton Ridge. The ridges and valleys of the Diablo Range, containing the county's highest peaks, cover the very sparsely populated southeast portion of the county.
The Hayward Fault, a major branch of the San Andreas Fault to the west, runs through the most populated parts of Alameda County, while the Calaveras Fault runs through the southeastern part of the county.
The areas near the Bay itself have a maritime warm-summer Mediterranean climate, whereas behind the mountains, summers are significantly warmer. The climate charts below are for Oakland and inland Livermore.
Adjacent counties
The City and County of San Francisco, California, borders the county on the west, and has a small land border with the city of Alameda, California due to land filling.
Santa Clara County borders the county on the south.
San Joaquin County borders the county on the east.
Contra Costa County borders the county on the north.
Stanislaus County borders the county on the easternmost end of its southern boundary for .
National protected area
- Don Edwards San Francisco Bay National Wildlife Refuge (part)