While most of the towns in the surrounding San Fernando Valley agreed to annexation by Los Angeles in the 1910s, eager to tap the bountiful water supply provided by the newly opened Los Angeles Aqueduct, San Fernando's abundant groundwater supplies allowed it to remain a separate city. Even as the San Fernando Valley transformed itself from an agricultural area to a suburban one in the decades after World War II, San Fernando retained its independence.
As with much of the San Fernando Valley east of the San Diego Freeway, the city of San Fernando has seen a significant demographic shift in recent years; declining birth-rates, an aging population of middle-class whites, who once dominated the area in the 1950s, has contributed to the movement into other parts of the San Fernando Valley. There has also been movement into the Santa Clarita and Antelope Valleys to the north. Income levels have declined as a result. Latinos became the majority. Since late 2004, the city has been going through a series of planning development projects, that can be academically identified as gentrification.
As of the census of 2000, there were 23,564 people, 5,774 households, and 4,832 families residing in the city. The population density was 3,822.7/km² (9,880.7/mi²). There were 5,932 housing units at an average density of 962.3/km² (2,487.4/mi²). The racial makeup of the city was 42.76% White, 0.98% African American, 1.69% Native American, 1.12% Asian, 0.11% Pacific Islander, 49.35% from other races, and 3.98% from two or more races. Hispanic or Latino of any race were 89.28% of the population.
There were 5,774 households out of which 52.8% had children under the age of 18 living with them, 59.1% were married couples living together, 16.4% had a female householder with no husband present, and 16.3% were non-families. 12.4% of all households were made up of individuals and 5.6% had someone living alone who was 65 years of age or older. The average household size was 4.07 and the average family size was 4.33.
In the city the population was spread out with 34.4% under the age of 18, 11.4% from 18 to 24, 32.1% from 25 to 44, 15.0% from 45 to 64, and 7.0% who were 65 years of age or older. The median age was 27 years. For every 100 females there were 101.7 males. For every 100 females age 18 and over, there were 99.9 males.
The median income for a household in the city was $39,909, and the median income for a family was $40,138. Males had a median income of $26,068 versus $22,599 for females. The per capita income for the city was $11,485. 15.3% of families and 19.1% of the population were below the poverty line, including 22.5% of those under age 18 and 15.6% of those age 65 or over.