As of the census of 2000, there were 54,977 people, 18,111 households, and 13,221 families residing in the city. The population density was 893.4/km² (2,314.3/mi²). There were 18,862 housing units at an average density of 306.5/km² (794.0/mi²). The racial makeup of the city was 67.39% White, 2.00% Black, 0.82% Native American, 4.67% Asian, 0.24% Pacific Islander, 20.39% from other races, and 4.48% from two or more races. Hispanic or Latino of any race were 36.87% of the population.
There were 18,111 households out of which 39.1% had children under the age of 18 living with them, 58.6% were married couples living together, 9.6% had a female householder with no husband present, and 27.0% were non-families. 20.3% of all households were made up of individuals and 10.6% had someone living alone who was 65 years of age or older. The average household size was 3.03 and the average family size was 3.46.
In the city the population was spread out with 29.1% under the age of 18, 9.4% from 18 to 24, 32.3% from 25 to 44, 17.3% from 45 to 64, and 11.9% who were 65 years of age or older. The median age was 32 years. For every 100 females there were 98.4 males. For every 100 females age 18 and over, there were 95.3 males.
The median income for a household in the city was $45,908, and the median income for a family was $51,292. Males had a median income of $36,297 versus $27,015 for females. The per capita income for the city was $18,657. About 7.8% of families and 12.0% of the population were below the poverty line, including 14.5% of those under age 18 and 7.9% of those age 65 or over.
According to estimates by the San Diego Association of Governments, the median household income of San Marcos in 2005 was $63,558 (not adjusted for inflation). When adjusted for inflation (1999 dollars; comparable to Census data above), the median household income was $51,611.