Born in Chester County, Thomas McKean found early success as a lawyer in the colony of Delaware and went on to become the second governor of Pennsylvania, one of many prestigious public offices he held.

McKean first attained public office at the age of 28, joining the Delaware Assembly in 1762. Soon afterward, he became an outspoken opponent of British policies in Colonial America and was chosen to represent three Delaware counties at the Stamp Act Congress held in New York in 1765. His service there solidified his public image as an advocate of change. As the colonies moved closer to revolution, McKean joined a Philadelphia militia unit, and on July 4, 1776, as a delegate to the Second Continental Congress, he signed the Declaration of Independence.

Shortly after taking office as chief justice of Pennsylvania's Supreme Court in August 1777, McKean served as president (governor) of Delaware for several weeks following the British capture of John McKinly, the previous president. He served in the Continental Congress throughout most of the American Revolution, and he was president of that body - the nation's highest elected official - during the critical months leading up to Lord Cornwallis' surrender at Yorktown.

McKean was a strong supporter of Thomas Jefferson's Democratic-Republican Party, and he was elected governor of Pennsylvania in 1799. Based on allegations that he abused his power, McKean faced a drive for his impeachment in 1808; however, his opponents lacked the necessary support to oust him. He left the governorship at the end of that year and retired from public life.