According to the United States Census Bureau, the CDP has a total area of 0.7 square miles (1.8 km²), all of it land.
Port Costa was founded in 1879 as a landing for the railroad ferry Solano, owned and operated by the Central Pacific Railroad. This put Port Costa on the main route of the transcontinental railroad. The Solano, later joined by the Contra Costa, carried entire trains across the Carquinez Strait from Benicia to Port Costa, from whence they continued on to the Oakland Pier. For a time, it was also the United States' busiest wheat-shipping port and had a reputation as a colorful, sometimes violent community.
After California's wheat output dropped in the early 20th Century and especially, after the Southern Pacific (which took over the operations of the Central Pacific) constructed a railroad bridge at Martinez in 1930 to replace the ferry crossing, Port Costa lost population and importance. Bill Rich was an influential property owner and raconteur. Since the late 1960s, it has mainly been a small shopping venue for antique hunters.
As of the census of 2000, there were 232 people, 108 households, and 60 families residing in the CDP. The population density was 342.1 people per square mile (131.7/km²). There were 115 housing units at an average density of 169.6/sq mi (65.3/km²). The racial makeup of the CDP was 90.95% White, 0% Black,1.29% Native American, 1.29% Asian, 1.72% from other races, and 4.74% from two or more races. 6.90% of the population were Hispanic or Latino of any race.
There were 108 households out of which 20.4% had children under the age of 18 living with them, 46.3% were married couples living together, 3.7% had a female householder with no husband present, and 44.4% were non-families. 34.3% of all households were made up of individuals and 8.3% had someone living alone who was 65 years of age or older. The average household size was 2.15 and the average family size was 2.80.
In the CDP the population was spread out with 15.5% under the age of 18, 4.7% from 18 to 24, 28.0% from 25 to 44, 37.5% from 45 to 64, and 14.2% who were 65 years of age or older. The median age was 46 years. For every 100 females there were 121.0 males. For every 100 females age 18 and over, there were 110.8 males.
The median income for a household in the CDP was $61,429, as was the median income for a family. Males had a median income of $40,769 versus $58,000 for females. The per capita income for the CDP was $33,563. About 9.7% of families and 10.7% of the population were below the poverty line, including 27.9% of those under the age of eighteen and none of those sixty five or over.
100% of the residents speak English.