McKean was born in Chester County, Pennsylvania on March 19, 1734. He became a Presbyterian minister, and read law after moving to New Castle, Delaware. From the age of eighteen he served in many official positions: deputy general; clerk of the provincial assembly; member of the assembly; judge of the court of common pleas; collector of customs for New Castle. At the age of twenty, he was admitted to the bar in Delaware, New Jersey and Pennsylvania.
McKean had opposed British rule since the 1765 Stamp Act; as a Delaware representative in the Continental Congress, he urged Delaware to vote for independence. Soon after the vote, he left Congress to lead a battalion in the Continental Army. He signed the Declaration of Independence at some later date; his name does not appear on the copy that was authenticated in January 1777. He was elected Governor of Pennsylvania in 1799, and served three terms.