Don Palomares received a 15,000 acre land grant from Governor Juan Bautista Alvarado in 1837, which included the land of present day cities Pomona, Claremont, La Verne, Glendora, and San Dimas. This holding was called Rancho San Jose. The northern part of the Rancho was cut through with wild canyons, and while they were of little use for vineyards, beans, and corn, they were an ideal place to house Don Palomares's prize cattle and horses during harsh winters. Due to their distance from the Hacienda, these corrals were often raided by horse thieves and cattle rustlers. Frustrated by the loss of his cattle, Don Palomares began referring to the area as "San Dismas", in hopes that the thieves would turn to Saint Dismas (the patron saint of reformed thieves) and repent of their crimes. In time, the name caught on, and the area was called San Dismas. Eventually, the first 's' was dropped, and the area became known as San Dimas; such a dropping of syllable-final [s] is actually common in many forms of Spanish language.
As of the censusGR2 of 2000, there were 34,980 people, 12,163 households, and 8,988 families residing in the city. The population density was 870.8/km² (2,255.7/mi²). There were 12,503 housing units at an average density of 311.2/km² (806.3/mi²). The racial makeup of the city was 74.66% White, 3.30% African American, 0.69% Native American, 9.39% Asian, 0.21% Pacific Islander, 7.34% from other races, and 4.39% from two or more races. 23.34% of the population were Hispanic or Latino of any race.
There are 12,163 households out of which 35.5% have children under the age of 18 living with them, 57.7% are married couples living together, 11.6% have a female householder with no husband present, and 26.1% are non-families. 21.0% of all households are made up of individuals and 8.7% have someone living alone who is 65 years of age or older. The average household size is 2.78 and the average family size is 3.23.
In the city the population was spread out with 25.5% under the age of 18, 8.9% from 18 to 24, 28.1% from 25 to 44, 25.5% from 45 to 64, and 11.9% who are 65 years of age or older. The median age is 37 years. For every 100 females there were 92.2 males. For every 100 females age 18 and over, there were 87.2 males.
The median income for a household in the city was $62,885, and the median income for a family was $72,124. Males had a median income of $53,009 versus $36,057 for females. The per capita income for the city was $28,321. 6.3% of the population and 3.6% of families were below the poverty line. Out of the total population, 5.9% of those under the age of 18 and 11.5% of those 65 and older were living below the poverty line.