San Gabriel Valley is in Los Angeles County. The incorporated cities and unincorporated neighborhoods of the San Gabriel Valley include:

Forbes "Most Expensive ZIP Codes" published in 2008 features nine ZIPs within the San Gabriel Valley that rank within the top 500 cities with the most expensive homes in the United States.

Whittier is considered[by whom?]both a San Gabriel Valley city and part of the Gateway Cities region. Some of Whittier sits below the Whittier Narrows. Although these hills are small compared to the San Gabriel Mountains, the fact that most of the city sits around them is said to make Whittier a San Gabriel Valley city, despite its area codes—most of Whittier is served by the 562 area code, though a large portion is served by the 626 area code, with which the valley is primarily associated. This is similar to Montebello, which is situated in the 323 area code and is a member of the Gateway Cities Council of Governments, despite geographically being part of the San Gabriel Valley.

San Dimas, Claremont, Pomona, Diamond Bar, and La Verne are adjacent to the San Gabriel Valley, and although are properly considered part of the Pomona Valley, they are also commonly considered[by whom?] as part of the San Gabriel Valley. They share the 909 area code with other cities in the San Bernardino Valley portion of the Inland Empire. The 57 Freeway (Orange Freeway) is generally considered[by whom?] the dividing line between the Pomona and San Gabriel Valleys. However, for statistical and economic development purposes, the County of Los Angeles generally includes these five cities as part of the San Gabriel Valley.[citation needed]

The community of El Sereno, in the city of Los Angeles, is at the westernmost edge of the Valley.

Unofficial estimates[citation needed] place the combined population of the San Gabriel Valley at around 2 million—roughly a fifth of the population of Los Angeles County.

The majority of people residing in the San Gabriel Valley are Hispanics and Asian Americans.[citation needed] The white population in the San Gabriel Valley resides primarily in the communities of Pasadena, South Pasadena, Glendora, Monrovia, Sierra Madre, Charter Oak, and in the southern and eastern parts of Covina.[citation needed] Significant percentages of all major ethnic groups, however, reside in most San Gabriel Valley communities, and the area is in general one of the most ethnically diverse regions in the country.

The African American population in the San Gabriel Valley is relatively low, particularly when compared to that of more central Greater Los Angeles communities like Inglewood. However, there are sizable, long-established African American communities in the western Altadena area and in northwest Pasadena, as well as in Monrovia.

Hispanics, mainly Mexican Americans, are especially dominant in Azusa, Baldwin Park, City of Industry, El Monte, La Puente, Montebello, Rosemead, South El Monte, West Covina, and Whittier.[citation needed] Mexican Americans have been present in the area since the 1840s, when the U.S. government obliged Mexico to cede this territory after the Mexican-American War. In the predominantly Asian American city of Monterey Park, Hispanics are concentrated in the southwestern part of the city[citation needed] near East Los Angeles and the Belvedere district of Los Angeles. The southwestern portion was formerly East Los Angeles before annexation by Monterey Park years ago.

The original Native American group in the area are the Tongva tribe, who are also known as the Gabrielino (or Gabrieleño in Spanish), due to their association with the mission in San Gabriel.