Butler County was created on March 12, 1800, from part of Allegheny County and named in honor of General Richard Butler. Its county seat is the city of Butler.
As of the census of 2000, there were 174,083 people, 65,862 households, and 46,827 families residing in the county. The population density was 221 people per square mile (85/km²). There were 69,868 housing units at an average density of 89 per square mile (34/km²). The racial/ethnic makeup of the county is 97.83% White, 0.79% Black or African American, 0.09% Native American, 0.56% Asian, 0.03% Pacific Islander, 0.17% from other races, 0.54% from two or more races; and 0.58% Hispanic or Latino of any race. 34.8% were of German, 12.0% Irish, 10.9% Italian, 7.0% English, 6.3% American and 5.2% Polish ancestry according to Census 2000.
There were 65,862 households out of which 32.90% had children under the age of 18 living with them, 59.80% were married couples living together, 8.10% had a female householder with no husband present, and 28.90% were non-families. 24.20% of all households were made up of individuals and 10.40% 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 3.04.
In the county, the population was spread out with 24.60% under the age of 18, 8.80% from 18 to 24, 29.40% from 25 to 44, 23.00% from 45 to 64, and 14.30% who were 65 years of age or older. The median age was 38 years. For every 100 females there were 95.40 males. For every 100 females age 18 and over, there were 91.80 males.