As of the census of 2000, there were 799,407 people, 252,940 households, and 186,669 families residing in the county. The population density was 134 people per square mile (52/km²). There were 270,767 housing units at an average density of 45 per square mile (18/km²). The racial makeup of the county was 54.30% White, 5.30% Black or African American, 1.60% Native American, 8.05% Asian, 0.13% Pacific Islander, 25.90% from other races, and 4.73% from two or more races. 43.99% of the population were Hispanic or Latino of any race. 7.5% were of German ancestry according to Census 2000. 59.3% spoke English, 31.5% Spanish and 3.1% Hmong as their first language.
There were 252,940 households out of which 41.20% had children under the age of 18 living with them, 52.50% were married couples living together, 15.20% had a female householder with no husband present, and 26.20% were non-families. 20.60% of all households were made up of individuals and 7.80% had someone living alone who was 65 years of age or older. The average household size was 3.09 and the average family size was 3.59.
In the county the population was spread out with 32.10% under the age of 18, 11.10% from 18 to 24, 28.50% from 25 to 44, 18.50% from 45 to 64, and 9.90% who were 65 years of age or older. The median age was 30 years. For every 100 females there were 100.40 males. For every 100 females age 18 and over, there were 98.20 males.
The median income for a household in the county was $34,725, and the median income for a family was $38,455. Males had a median income of $33,375 versus $26,501 for females. The per capita income for the county was $15,495. About 17.60% of families and 22.90% of the population were below the poverty line, including 31.70% of those under age 18 and 9.90% of those age 65 or over.
Fresno County is also known for having the highest rate of chlamydia in the state, which in in 2006 545.2 cases per 100,000 people, compared with the statewide average of 363.5.