In 1798, John Carlysle Stewart, a civil engineer, traveled to western Pennsylvania to resurvey the "donation lands" granted by the government to revolutionary war veterans. In the course of performing his task, he discovered that the original survey forgot to stake out 50 acres at the confluence of the Shenango River and Neshannock Creek. Stewart claimed it for himself. There and then was the beginning of the City of New Castle, Pennsylvania.

Stewart laid out the town of New Castle in April of 1798. It was comprised of approximately 50 acres in what was then part of Allegheny County. New Castle became a borough in 1825, having a population of about 300. The city later became a part of Mercer County. On April 5, 1849 the Governor of the Commonwealth of Pennsylvania signed an act creating Lawrence County. New Castle became a city in 1869 and was headed by its first Mayor, Thomas B. Morgan. At that time, the population had increased to about 6,000.