Thomas Mifflin (January 10, 1744 – January 20, 1800) was an American merchant, soldier, and politician from Pennsylvania, who is regarded as a Founding Father of the United States for his roles during and after the American Revolution. Mifflin signed the United States Constitution, was the first governor of Pennsylvania, serving from 1790 to 1799, and was also the state's last president, succeeding Benjamin Franklin in 1788.

Born in Philadelphia, Mifflin became a merchant following his graduation from the College of Philadelphia. After serving in the Pennsylvania Provincial Assembly and the First Continental Congress, where he signed the Continental Association, he joined the Continental Army in 1775. During the Revolutionary War, Mifflin was an aide to General George Washington and was appointed the army's Quartermaster General, rising to the rank of major general. He returned to Congress in 1782 and was elected president of the Congress the following year. He served as speaker of the Pennsylvania House of Representatives from 1785 to 1787 and as president of Pennsylvania's Supreme Executive Council from 1788 to 1790.

Mifflin was a delegate to the Constitutional Convention in 1787 and signed the United States Constitution. He then presided over the committee that wrote Pennsylvania's state constitution, becoming the state's first governor after the constitution's ratification in 1790. Mifflin left office as governor in 1799 and died the following year.

Mifflin was born January 10, 1744, in Philadelphia, in the Province of Pennsylvania. He was the son of John Mifflin and Elizabeth Bagnall. His great-grandfather John Mifflin Jr. (1662–1714) was born in Warminster, Wiltshire, England and settled in the Province of Pennsylvania.

In 1760, Mifflin graduated from the College of Philadelphia (now the University of Pennsylvania) and joined the mercantile business of William Biddle. After returning from a trip to Europe in 1765, he established a commercial business partnership with his brother, George Mifflin.

He married a second cousin, Sarah Morris, on March 4, 1767. Their daughter Emily Mifflin married Joseph Hopkinson, the son of Francis Hopkinson. After Sarah's death in 1790, Emily became her father's hostess and a family source references a total of four daughters in the family, "all beautiful women." A source indicates that Sarah "had no children of her own."

Early in the Revolutionary War, Mifflin left the Continental Congress to serve in the Continental Army. He was commissioned as a major, then became an aide-de-camp of George Washington.

On August 14, 1775, Washington appointed him to become the army's first quartermaster general, under order of Congress. Although it has been said that he was good at the job despite preferring to be on the front lines, questions were raised regarding his failure to properly supply Washington and the troops at Valley Forge, alleging that he had instead warehoused and sold supplies intended for Valley Forge to the highest bidder. Reportedly, after Washington confronted him about this, Mifflin asked to be relieved as quartermaster general but was persuaded to resume those duties because Congress was having difficulty finding a replacement.