The county seat is Kittanning. The county was organized on March 12, 1800 from parts of Allegheny, Westmoreland and Lycoming Counties. It was named in honor of John Armstrong, who had represented Pennsylvania in the Continental Congress.
As of the census of 2000, there were 72,392 people, 29,005 households, and 20,535 families residing in the county. The population density was 111 people per square mile (43/km²). There were 32,387 housing units at an average density of 50 per square mile (19/km²). The racial makeup of the county was 98.32% White, 0.82% Black or African American, 0.09% Native American, 0.12% Asian, 0.02% Pacific Islander, 0.13% from other races, and 0.50% from two or more races. 0.43% of the population were Hispanic or Latino of any race. 34.6% were of German, 10.8% Italian, 9.3% Irish, 8.7% American, 7.4% English and 5.7% Polish ancestry according to Census 2000.
There were 29,005 households out of which 29.50% had children under the age of 18 living with them, 57.90% were married couples living together, 9.00% had a female householder with no husband present, and 29.20% were non-families. 25.90% of all households were made up of individuals and 13.70% had someone living alone who was 65 years of age or older. The average household size was 2.46 and the average family size was 2.95.
In the county, the population was spread out with 22.90% under the age of 18, 7.20% from 18 to 24, 27.60% from 25 to 44, 24.20% from 45 to 64, and 18.00% who were 65 years of age or older. The median age was 40 years. For every 100 females there were 94.70 males. For every 100 females age 18 and over, there were 92.10 males.