The Spanish word alameda means either, "...a grove of poplars,...or a tree lined street" a name 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. Although a strict translation to English might be "Poplar Grove Creek", the name of the principal stream that flows through the county is now simply "Alameda Creek."
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 1872–1875. 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 to multiple city centers and suburbs, is shared with the adjacent and closely associated Contra Costa County.
The annual county fair is held at the Alameda County Fairgrounds in Pleasanton. The fair runs for four weekends from June to July. Attractions include horse racing, carnival rides, 4-H exhibits, and live bands.
According to the U.S. Census Bureau, the county has a total area of 821 square miles (2,130 km2), of which 739 square miles (1,910 km2) is land and 82 square miles (210 km2) (10%) is water. The San Francisco Bay borders the county on the west.
The crest of the Berkeley Hills form part of the northeastern boundary and reach 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 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 2010 United States Census reported that Alameda County had a population of 1,510,271. The population density was 2,047.6 people per square mile (790.6/km2). The racial makeup of Alameda County was 649,122 (43.0%) White, 190,451 (12.6%) African American, 9,799 (0.6%) Native American, 394,560 (26.1%) Asian (9.7% Chinese, 5.5% Filipino, 4.8% Indian, 2.0% Vietnamese, 1.2% Korean, 0.8% Japanese, 2.2% Other Asian), 12,802 (0.8%) Pacific Islander, 162,540 (10.8%) from other races, and 90,997 (6.0%) from two or more races. Hispanic or Latino of any race were 339,889 persons (22.5%): 16.4% Mexican, 0.8% Puerto Rican, 0.2% Cuban, 5.1% Other Hispanic.
As of the census of 2000, there were 1,443,741 people, 523,366 households, out of which 32.6% had children under the age of 18 living within them, 47.0% married couples living together, 13.0% had a female householder with no husband present, and 35.2% were non-families. 26.0% of all households were made up of individuals, and 7.3% had someone living alone who was 65 years of age or older. The average household size was 2.71 and the average family size was 3.31.
In the county, the population was spread out, with 24.6% under the age of 18, 9.6% from 18 to 24, 33.9% from 25 to 44, 21.7% from 45 to 64, and 10.2% who were 65 years of age or older. The median age was 34 years. For every 100 females there were 96.60 males. For every 100 females age 18 and over, there were 94.00 males.
The median income for a household in the county was $55,946, and the median income for a family was $65,857 (these figures had risen to $66,430 and $81,341 respectively as of a 2007 estimate). Males had a median income of $47,425 versus $36,921 for females. The per capita income for the county was $26,680. About 7.7% of families and 11.0% of the population were below the poverty line, including 13.5% of those under age 18 and 8.1% of those age 65 or over.
In 2000, the largest denominational group was the Catholics (with 306,437 adherents) . The largest religious bodies were the Catholic Church (with 306,437 members) and Judaism (with 32,500 members).
2018 United States Census American Community Survey estimates
The White population continues to remain the largest racial category in Alameda County and includes the 34.4% of Hispanics who self-identify as White. The remainder of Hispanics self-identify as Some Other Race (53.0%), Multiracial (9.5%), American Indian and Alaskan Native (1.3%), Black (0.9%), Asian (0.7%), and Hawaiian and Pacific Islander (0.1%).
The Black population continues to decline and at 10.3% (including Black Hispanics) is below the national average of 12.7% (including Black Hispanics). The Black population peaked in the 1980 Census at 18.4%. Alameda county has the 2nd highest percentage of Black residents in California after Solano County at 13.4%.
Asian Americans remain the largest minority group at either 30.8% (including Asian Hispanics) or 30.6% (excluding Asian Hispanics).
By ethnicity, 22.4% of the total population is Hispanic-Latino (of any race) and 77.6% is Non-Hispanic (of any race). If treated as a category separate from race, Hispanics are the third largest minority group in Alameda County.
The largest ancestry group of Hispanics in Alameda County are of Mexican descent (72.9% of Hispanics) followed by Salvadoran descent (5.5% of Hispanics), Guatemalan descent (3.9%), Puerto Rican descent (3.4%), Spaniard descent (2.0%), Nicaraguan descent (1.7%), Peruvian descent (1.4%), Cuban descent (1.2%), Colombian descent (1.1%), and those of other Hispanic ethnicity or of mixed Hispanic ethnicity (6.9%).
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.
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 Miley is serving currently as president; Supervisor Carson 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 , who was elected in 2006, succeeding Charles Plummer, who had served in the post for 20 years. The Interim District Attorney is , who was appointed to fill the position of retiring District Attorney Tom Orloff in September 2009. The Sheriff's Office operates two jails: Santa Rita Jail in Dublin, and in downtown Oakland.
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 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.
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 slightly larger margin. In 2004 it voted for John Kerry, who won over 75% of the vote. Every city and town voted Democratic. Geoge W Bush in 2004 was the last Republican to break 20% of the county's vote, his father (George H.W. Bush) in 1988 was the last to break 30% of the vote, and Ronald Reagan in 1984 was the last to break 40% of the vote (carrying 40.01%). 
Presidential election results
Alameda County vote by party in presidential elections
The California Secretary of State, as of February 2019, reports that there are 883,942 registered voters in Alameda County. 489,759 (55.4%) are registered Democrats, 95,587 (10.8%) are registered Republicans, 36,649 (4.1%) are registered to minor political parties, and 261,947 (29.6%) declined to answer. Every city, town, and unincorporated area in Alameda County has more registered Democrats than Republicans.
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.
The Alameda County Office of Education oversees seventeen K–12 school districts and one K–8 district in Alameda County. In all, there are approximately 10,000 teachers serving 225,000 students. The ACOE also services three community college districts with a total enrollment of approximately 55,000 students.
The , a division of the county administration, under the California Arts Council, was created in 1965. Its fifteen appointed members act in an advisory capacity to the board of supervisors, in promoting the arts. The Oakland Museum of California has a substantial collection of California art works and historical artifacts.
The following sports teams play in Alameda County:
ACE train – commuter rail using existing railroad tracks; primarily brings commuters from San Joaquin County to Santa Clara County
AC Transit – local bus system in western Alameda County and west Contra Costa County, with additional service across the three bridges from Alameda County to downtown San Francisco, San Mateo, and Palo Alto
BART – commuter rail centered on northwest Oakland, primarily serving commuters to downtown San Francisco and downtown Oakland
Capitol Corridor – commuter rail using existing railroad tracks, extending from San Jose to Sacramento, running through western Alameda County
WHEELS – bus system in the cities of southeastern Alameda County