Allegheny County is a county in the southwestern part of the U.S. state of Pennsylvania. As of the 2010 census, the population was 1,223,348, making it the second most populous county in Pennsylvania following Philadelphia County. The county seat is Pittsburgh. The county forms the nucleus of the tri-state Pittsburgh metropolitan area and Pittsburgh DMA.
Allegheny County was the first in Pennsylvania to be given a Native American name, being named after the Allegheny River. The word "Allegheny" is of Lenape origin, with uncertain meaning. It is usually said to mean "fine river", but sometimes said to refer to an ancient mythical tribe called "Allegewi" who lived along the river long ago before being destroyed by the Lenape.
Not a great deal is known about the native inhabitants of the region prior to European contact. During the colonial era various native groups claimed or settled in the area, resulting in a multi-ethnic mix that included Iroquois, Lenape, Shawnee, and Mingo.
The first Europeans to enter the area were the French in 1749. Captain Pierre Joseph de Celeron, sieur de Blainville claimed the Ohio Valley and all of western Pennsylvania for Louis XV of France. The captain traveled along the Ohio and Allegheny rivers inserting lead plates in the ground to mark the land for France.
Since most of the towns during that era were developed along waterways, both the French and the British desired control over the local rivers. Therefore, the British sent Major George Washington to try to compel the French to leave their posts, with no success. Having failed in his mission, he returned and nearly drowned crossing the ice-filled Allegheny River. In 1754, the English tried again to enter the area. This time, they sent 41 Virginians to build Fort Prince George. The French got news of the plan and sent an army to take over the fort, which they then resumed building with increased fortification, renaming it Fort Duquesne.
The loss of the fort cost the English dearly because Fort Duquesne became one of the focal points of the French and Indian War. The first attempt to retake the fort, the Braddock Expedition, failed miserably. It was not until General John Forbes attacked in 1758, four years after they had lost the original fort, that they recaptured and destroyed it. They subsequently built a new fort on the site, including a moat, and named it Fort Pitt. The site is now Pittsburgh's Point State Park.
Both Pennsylvania and Virginia claimed the region that is now Allegheny County. Pennsylvania administered most of the region as part of its Westmoreland County. Virginia considered everything south of the Ohio River and east of the Allegheny River to be part of its Yohogania County and governed it from Fort Dunmore. In addition, parts of the county were located in the proposed British colony of Vandalia and the proposed U.S. state of Westsylvania. The overlapping boundaries, multiple governments, and confused deed claims soon proved unworkable. In 1780 Pennsylvania and Virginia agreed to extend the Mason–Dixon line westward, and the region became part of Pennsylvania. From 1781 until 1788, much of what had been claimed as part of Yohogania County, Virginia, was administered as a part of the newly created Washington County, Pennsylvania.
Allegheny County was officially created on September 24, 1788, from parts of Washington and Westmoreland counties. It was formed due to pressure from settlers living in the area around Pittsburgh, which became the county seat in 1791. The county originally extended all the way north to the shores of Lake Erie and became the "mother county" for most of what is now northwestern Pennsylvania. By 1800, the county's current borders were set.