Important Pennsylvanians of both dominant political parties emerged as leaders of the Revolutionary movement—Benjamin Franklin, Benjamin Rush, Joseph Reed, Thomas Mifflin, John Dickinson, Robert Morris, and Haym Salomon. In 1776 a provincial convention dominated by radical patriots created the Commonwealth of Pennsylvania under one of the most democratic of the new state constitutions.

The state was invaded by British troops, and notable engagements were fought in 1777 on the Brandywine and at Germantown. Philadelphia was occupied by the British, while Valley Forge witnessed the heroic endurance of Washington’s troops in the winter of 1777–78, making the site a shrine of patriotism. In the postwar period, Pennsylvania’s role as the geographical keystone of the new nation was strengthened by its resolution of boundary disputes that had persisted throughout the colonial period: agreement was reached with Maryland in 1784 by acceptance of the Mason-Dixon line; with Virginia and New York in 1786; with the United States and the Iroquois Confederacy in 1789; and with Connecticut in 1799 after bitter dissension in the Wyoming Valley.

Philadelphia, host to the First and Second Continental Congresses (1774, 1775–81) and scene of the signing of the Declaration of Independence, was for many years the nation’s leading city. It was the site of the Constitutional Convention of 1787, served as the seat of the new federal government from 1790 to 1800, and became a financial center through the organization of the First Bank of the United States (1791) and the U.S. Mint (1792). In 1790 it was also the site of a convention that replaced the radical state constitution of 1776 with a more conservative one patterned after the federal Constitution, while retaining such liberal achievements as the act (1780) providing for the gradual abolition of slavery. Philadelphia was not, however, typical of the state as a whole.