 Arnold's only land border is with New Kensington to the north, east and south.Across the Allegheny River, Arnold runs adjacent with East Deer Township in Allegheny County

As of the census of 2000, there were 5,667 people, 2,589 households, and 1,439 families residing in the city.The population density was 7,706.6 people per square mile (2,956.8/km).