Arnold is a city in Westmoreland County, Pennsylvania, United States, within the Pittsburgh metropolitan area. The population was 5,157 at the 2010 Census.
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. William Jack acquired the property and later passed it on to Wilson Jack. The area around Arnold was first settled in 1852 by Major Andrew Arnold, who grew up in Kittanning.
With the creation of Westmoreland County in 1773, the area that became Arnold was originally part of Burrell, and later Lower Burrell, townships. Arnold was then part of the newly-created city of New Kensington from 1891 until it was separately incorporated as a borough in January 1896, and as a third-class city in 1939. 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).