As of the census of 2000, there were 1,281,666 people, 537,150 households, and 332,495 families residing in the county. The population density was 1,755 people per square mile (678/km²). There were 583,646 housing units at an average density of 799 per square mile (309/km²). The racial makeup of the county was 84.33% White, 12.41% Black or African American, 0.12% Native American, 1.69% Asian, 0.03% Pacific Islander, 0.34% from other races, and 1.07% from two or more races. About 0.87% of the population were Hispanic or Latino of any race. 20.0% were of German, 15.0% Italian, 12.7% Irish, 7.5% Polish and 5.1% English ancestry according to Census 2000. 93.5% spoke English and 1.3% Spanish as their first language.
There were 537,150 households out of which 26.40% had children under the age of 18 living with them, 46.10% were married couples living together, 12.40% had a female householder with no husband present, and 38.10% were non-families. Some 32.70% of all households were made up of individuals and 13.20% had someone living alone who was 65 years of age or older. The average household size was 2.31 and the average family size was 2.96.
The population was spread out with 21.90% under the age of 18, 8.50% from 18 to 24, 28.30% from 25 to 44, 23.40% from 45 to 64, and 17.80% who were 65 years of age or older. The median age was 40. For every 100 females, there were 90.00 males; for every 100 females age 18 and over, there were 86.20 males.
Allegheny County's public transportation provider is the Port Authority of Allegheny County.
For information about roads, see list of State Routes in Allegheny County, Pennsylvania and Pittsburgh/Allegheny County Belt System.