According to the United States Census Bureau, the CDP has a total area of 3.1 square miles (7.9 km²), all of it land.
As of the census of 2000, there were 3,017 people, 1,293 households, and 812 families residing in the CDP. The population density was 987.0 people per square mile (380.7/km²). There were 1,446 housing units at an average density of 473.0/sq mi (182.5/km²). The racial makeup of the CDP was 91.48% White, 0.10% African American, 2.22% Native American, 1.19% Asian, 0.27% Pacific Islander, 1.92% from other races, and 2.82% from two or more races. Hispanic or Latino of any race were 9.48% of the population.
There were 1,293 households out of which 26.2% had children under the age of 18 living with them, 45.9% were married couples living together, 13.1% had a female householder with no husband present, and 37.2% were non-families. 31.1% of all households were made up of individuals and 18.1% had someone living alone who was 65 years of age or older. The average household size was 2.27 and the average family size was 2.84.
In the CDP the population was spread out with 23.0% under the age of 18, 7.4% from 18 to 24, 22.2% from 25 to 44, 23.2% from 45 to 64, and 24.2% who were 65 years of age or older. The median age was 42 years. For every 100 females there were 87.4 males. For every 100 females age 18 and over, there were 79.5 males.
The median income for a household in the CDP was $25,847, and the median income for a family was $34,896. Males had a median income of $27,083 versus $26,250 for females. The per capita income for the CDP was $16,209. About 14.1% of families and 16.3% of the population were below the poverty line, including 23.3% of those under age 18 and 5.5% of those age 65 or over.
In the state legislature Jamestown is located in the 14th Senate District, represented by Republican Dave Cogdill, and in the 25th Assembly District, represented by Republican Tom Berryhill. Federally, Jamestown is located in California's 19th congressional district, which has a Cook PVI of R +10 and is represented by Republican George Radanovich.