As of the census of 2000, there were 373,638 people, 141,570 households, and 98,532 families residing in the county. The population density was 435 people per square mile (168/km²). There were 150,222 housing units at an average density of 175 per square mile (68/km²). The racial makeup of the county in 2004 was 82.5% White, 4.6% Black or African American, 0.2% Native American, 1.2% Asian, 0.02% Pacific Islander, 6.3% from other races, and 1.2% from two or more races. Hispanics/Latinos made up 11.8%. 35.1% were of German, 8.6% Italian, 7.1% Irish, and 5.4% Polish ancestry according to Census 2000. 87.6% spoke English, 8.1% Spanish and 1.0% Pennsylvania Dutch as their first language. Historically there was a large Pennsylvania Dutch (from Pennsylvania Deutsch or German) population. It is known as part of Pennsylvania Dutch Country.
There were 141,570 households out of which 31.70% had children under the age of 18 living with them, 55.50% were married couples living together, 9.90% had a female householder with no husband present, and 30.40% were non-families. 24.60% of all households were made up of individuals and 10.70% had someone living alone who was 65 years of age or older. The average household size was 2.55 and the average family size was 3.05.
In the county, the population was spread out with 24.60% under the age of 18, 8.80% from 18 to 24, 28.90% from 25 to 44, 22.60% from 45 to 64, and 15.00% who were 65 years of age or older. The median age was 37 years. For every 100 females there were 95.90 males. For every 100 females age 18 and over, there were 92.70 males.