As of the census of 2000, there were 948,816 people, 344,129 households, and 242,266 families residing in the county. The population density was 1,318 people per square mile (509/km²). There were 354,577 housing units at an average density of 492 per square mile (190/km²). The racial makeup of the county was:
17.68% of the population were Hispanic or Latino of any race. 9.0% were of German, 7.7% Irish, 7.3% English and 6.5% Italian ancestry according to Census 2000. 74.1% spoke English, 13.1% Spanish, 2.6% Tagalog and 1.8% Chinese or Mandarin as their first language.
By 2005 53.2% of Contra Costa County's population was non-Hispanic whites. African-Americans made up 9.6% of the population, while Asians constituted 13.1% of it. Latinos were now 21.1% of the county population.
In 2000 there were 344,129 households out of which 35.4% had children under the age of 18 living with them, 54.5% were married couples living together, 11.5% had a female householder with no husband present, and 29.6% were non-families. 22.9% of all households were made up of individuals and 8.00% had someone living alone who was 65 years of age or older. The average household size was 2.72 and the average family size was 3.23.
In the county the population was spread out with:
The median age was 36 years. For every 100 females there were 95.4 males. For every 100 females age 18 and over, there were 92.2 males.
The median income for a household in the county was $63,675, and the median income for a family was $73,039 (these figures had risen to $75,483 and $87,435 respectively as of a 2007
estimate).
Males had a median income of $52,670 versus $38,630 for females. The per capita income for the county was $30,615. About 5.4% of families and 7.6% of the population were below the poverty line, including 9.8% of those under age 18 and 6.0% of those age 65 or over.
Prior to 1903 most travel to central Contra Costa County was by boat or rail to Martinez on the northern waterfront and from there to the industrial areas east along the waterfront as well as farming regions to the south.
In 1903 the first tunnel through the Oakland hills (now Old Tunnel Road) was built, principally as a means of bringing hay by horse, mule, or ox-drawn wagons from central and eastern agricultural areas to feed the draft animals that provided the power to public and private transportation in the East Bay at the time. The tunnel exited in the hills high above the crossroads of Orinda with the road continuing on to Lafayette, Walnut Creek, and Danville. The road was just wide enough for one car in each direction, and had no shoulders.
In 1937 the two-bore Caldecott Tunnel for road vehicles was completed, making interior Contra Costa much more accessible. After World War II the tunnels allowed waves of development to proceed, oriented toward Oakland rather than the northern shoreline, and the northern shoreline cities began to decline. The tunnel has since been augmented with an additional bore, with the central bore reversed in direction to accommodate commute traffic. Owing to extensive reverse commuting and general increases in traffic, a fourth bore is being planned.
The county also has two airports that are not currently providing passenger service:
The western termini of several original transcontinental railroad routes have been located in Oakland, in Alameda County, Including Union Pacific, Southern Pacific, and Santa Fe railroads. From Oakland, there are two primary routes east:
Formed in 1909, the Oakland Antioch Railway was renamed the Oakland Antioch & Eastern Railway in 1911. It extended through a 3,400-foot (1,000 m) tunnel in the Oakland Hills, from Oakland to Walnut Creek, Concord and on to Bay Point.
The current owner of the Santa Fe Rairoad's assets, BNSF Railway has the terminus of its transcontinental route in Richmond. Originally built by the San Francisco and San Joaquin Valley Railroad in 1896, the line was purchased by the Atchison, Topeka and Santa Fe Railway shortly thereafter. The line leaves Richmond through industrial and residential parts of West County before striking due east through Franklin Canyon and Martinez on its way to Stockton, Bakersfield and Barstow.
These railroads spurred the development of industry in the county throughout the late 19th and early 20th centuries, particularly driving development of the Standard Oil (now Chevron) refinery and port complex in Richmond.
There were a large number of short lines in the county between the late 19th century and the early 20th century. The rights of way of a number of these railroads also served as utility rights of way, particularly for water service, and so were preserved, and in the late 20th century enhanced as walking, jogging, and bicycle riding trails in the central portion of the county.