Present-day Westmoreland County was part of the hunting reserves of the Iroquois Indians.

White colonists began to penetrate the area in the 1750s, and circa 1781 Robert McCrea purchased the land on which Arnold is situated.

Arnold is governed under Pennsylvania's third-class city code, with a mayor, treasurer, controller, and four council members elected at-large.

Past mayors of Arnold have been M. Frank Horne (1939-1960), Alfred Colaianni (1960-1963), Willie DeMao (1963-2004), John Campbell (2004–2012), and Larry Milito (2012-2016).

In 2018, during protests over the shooting of an unarmed African American teen by a white police officer in East Pittsburgh, Peconi authored a social media post that urged law enforcement to "bring the hoses" against the protesters.

Calls for her resignation resulted in city council formally requesting the state legislature to remove her from office.

Arnold is located on the eastern shore of the Allegheny River, northeast of Pittsburgh.

Hunt Elementary once served as Arnold's high school prior to the merger of the two districts.