According to the United States Census Bureau, the CDP has a total area of 8.3 square miles (21.4 km²), all of it land.
As of the census of 2000, there were 610 people, 217 households, and 182 families residing in the CDP. The population density was 73.8 people per square mile (28.5/km²). There were 229 housing units at an average density of 27.7/sq mi (10.7/km²). The racial makeup of the CDP was 95.41% White, 0.16% Native American, 1.15% Asian, 0.66% from other races, and 2.62% from two or more races. Hispanic or Latino of any race were 2.79% of the population.
There were 217 households out of which 34.1% had children under the age of 18 living with them, 75.6% were married couples living together, 5.1% had a female householder with no husband present, and 16.1% were non-families. 12.4% of all households were made up of individuals and 4.6% had someone living alone who was 65 years of age or older. The average household size was 2.81 and the average family size was 3.05.
In the CDP the population was spread out with 26.4% under the age of 18, 5.4% from 18 to 24, 20.3% from 25 to 44, 33.3% from 45 to 64, and 14.6% who were 65 years of age or older. The median age was 44 years. For every 100 females there were 94.3 males. For every 100 females age 18 and over, there were 95.2 males.
The median income for a household in the CDP was $51,406, and the median income for a family was $53,000. Males had a median income of $41,917 versus $27,031 for females. The per capita income for the CDP was $27,218. About 5.9% of families and 7.9% of the population were below the poverty line, including 12.1% of those under age 18 and 2.7% of those age 65 or over.
In the state legislature Millville is located in the 4th Senate District, represented by Republican Sam Aanestad, and in the 2nd Assembly District, represented by Republican Doug LaMalfa. Federally, Millville is located in California's 2nd congressional district, which has a Cook PVI of R +13 and is represented by Republican Wally Herger.