As of the census[4] of 2000, there were 404,119 people, 133,901 households, and 101,496 families residing in the county. The population density was 776 people per square mile (300/km²). There were 138,998 housing units at an average density of 267 per square mile (103/km²). The racial makeup of the county was 79.27% White, 5.76% Black or African American, 0.31% Native American, 1.81% Asian, 0.04% Pacific Islander, 10.61% from other races, and 2.21% from two or more races. 23.74% of the population were Hispanic or Latino of any race. 20.1% were of German, 8.7% Irish, 5.5% Polish, 5.5% Italian and 5.2% English ancestry according to Census 2000. 75.1% spoke English and 21.1% Spanish as their first language.

There were 133,901 households out of which 41.60% had children under the age of 18 living with them, 61.20% were married couples living together, 10.00% had a female householder with no husband present, and 24.20% were non-families. 19.60% of all households were made up of individuals and 6.70% had someone living alone who was 65 years of age or older. The average household size was 2.97 and the average family size was 3.43.

In the county the population was spread out with 30.30% under the age of 18, 9.10% from 18 to 24, 31.90% from 25 to 44, 20.40% from 45 to 64, and 8.40% who were 65 years of age or older. The median age was 32 years. For every 100 females there were 101.20 males. For every 100 females age 18 and over, there were 98.80 males.

The median income for a household in the county was $59,351, and the median income for a family was $66,558. Males had a median income of $45,787 versus $30,013 for females. The per capita income for the county was $24,315. About 4.90% of families and 6.70% of the population were below the poverty line, including 8.80% of those under age 18 and 5.00% of those age 65 or over.