According to the United States Census Bureau, the city has a total area of 7.5 km² (2.9 mi²), all land. Some areas in El Monte are mistaken for South El Monte, but the majority of the area is actually the city of El Monte and only certain small portions located at the beginning and the end of the north side and south side which are North and South El Monte.
As of the census of 2000, there were 21,144 people, 4,620 households, and 4,088 families residing in the city. The population density was 2,834.6/km² (7,330.8/mi²). There were 4,724 housing units at an average density of 633.3/km² (1,637.8/mi²). The racial makeup of the city was 40.61% White, 0.38% African American, 1.57% Native American, 8.43% Asian, 0.18% Pacific Islander, 43.98% from other races, and 4.84% from two or more races. Hispanic or Latino of any race were 86.03% of the population.
There were 4,620 households out of which 52.0% had children under the age of 18 living with them, 61.0% were married couples living together, 17.5% had a female householder with no husband present, and 11.5% were non-families. 8.0% of all households were made up of individuals and 3.9% had someone living alone who was 65 years of age or older. The average household size was 4.57 and the average family size was 4.61.
In the city the population was spread out with 33.5% under the age of 18, 13.0% from 18 to 24, 31.2% from 25 to 44, 15.0% from 45 to 64, and 7.2% who were 65 years of age or older. The median age was 27 years. For every 100 females there were 105.5 males. For every 100 females age 18 and over, there were 104.0 males.
The median income for a household in the city was $24,656, and the median income for a family was $34,349. Males had a median income of $21,075 versus $18,949 for females. The per capita income for the city was $10,130. About 16.2% of families and 19.0% of the population were below the poverty line, including 22.7% of those under age 18 and 11.0% of those age 65 or over.
In the state legislature South El Monte is located in the 24th Senate District, represented by Democrat Gloria Romero, and in the 49th Assembly District, represented by Democrat Mike Eng. Federally, South El Monte is located in California's 32nd congressional district, which has a Cook PVI of D +17 and is represented by Democrat [[Judy Chu, who was elected to replace Hilda Solis, who resigned to take office as the United States Secretary of Labor in the administration of President Barack Obama.
South El Monte is served by Los Angeles County for law enforcement (the LASD-Temple City Station) and fire services.