During his 50-year political career, he took almost every position possible. From being the deputy county attorney to President of the United States, he contributed much of his time into the development of the government in both his home state of Delaware and the nation. He proposed the voting procedure that Congress adopted at the Stamp Act congress: that each colony, no matter the size or population, have one vote. In 1765 as county judge, he defied the British by not working with the papers that did not show the hated stamps.

Eleven years later in 1776, he joined with Caesar Rodney to register Delaware's approval of the Declaration of Independence. McKean also helped draft and sign the Articles of Confederation during his tenure of service as President, from July 10, 1781 till November 4, 1782. He received a letter from the General of Washington in October 1787 telling him that the British had surrendered, following the Battle of Yorktown.

After forty years of public service when McKean turned sixty-five, he resigned he from his position of Chief Justice. In 1799 he was then elected the Governor of Pennsylvania. Thomas became the father of the spoils system in America because of his strict policy of appointing only fellow Republicans to office. Of any of the Founding Fathers, he completed one of the longest continuous careers of public service..