Thomas Mifflin was born in Philadelphia, Pennsylvania, on January 10, 1744, to a wealthy Quaker family. He was the son of John Mifflin, a successful merchant and local politician, and Elizabeth Bagnell. Mifflin received his early education at a Quaker school. He then went on to study at the University of Pennsylvania (then known as College of Pennsylvania), located in Philadelphia. After graduating from college, Mifflin worked with a wealthy Pennsylvania merchant in order to prepare for a mercantile career. Mifflin decided to travel to Europe, where most of his time was spent in France. When he returned from Europe, Mifflin became a business partner with his brother, George, but he soon realized his passion for politics. On March 4, at the age of twenty three, Mifflin married one of his cousins, Sarah Morris. In 1778, he was elected as a member of the American Philosophical Society, which was considered a high honor.

Mifflin's career in politics began on March 9, 1771, when he was appointed as a warden of Pennsylvania. He served on the Colonial Legislature from 1772 to 1774, where he used his position to fight British control over the colonies. Mifflin was a delegate at the first Continental Congress in 1774. After helping to recruit troops, he won appointment as a Major in the Continental Army. In the summer of 1775, Mifflin became aide-de-camp to George Washington and then was made Quartermaster General of the Continental Army. He continued to move up the ranks to Major General on February 19, 1777, but he resigned from this position in 1779 after facing criticism. From 1782 to 1784, Mifflin was a member of the Second Continental Congress, serving as its president from December 1783 to the following June. Mifflin was then chosen to take part in the Constitutional Convention in 1787, where he helped create and sign the United States Constitution. This contribution is Mifflin's major literary accomplishment, making him one of our nation's founding fathers.

After the signing of the Constitution, Mifflin continued his political career in the Pennsylvania Legislature. He soon succeeded Benjamin Franklin as president of the Supreme Executive Council from 1788 to 1790. In September 1790, a state constitution was drafted in a convention over which Mifflin presided, and he was chosen to be the first governor of Pennsylvania. During his term of office, Mifflin made appearances to the lower counties, where he publically addressed the militia regarding the insurrection of Pennsylvania. He held his office as Governor of Pennsylvania until 1799. Mifflin wrote many political speeches and letters throughout his career, which adds to his list of literary accomplishments.

Thomas Mifflin died in Lancaster, Pennsylvania, on January 20, 1800, at the age of 56. The Commonwealth of Pennsylvania has named Mifflin County after Thomas Mifflin, to honor its first governor and one of our nation's founding fathers. Many colleges throughout Pennsylvania, including The Pennsylvania State University, have paid tribute to him by naming buildings after him.