Thomas Mifflin then presided over the committee that wrote Pennsylvania's state constitution, becoming the state's first governor after the constitution's ratification in 1790. 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. 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 presided over the committee that wrote Pennsylvania's 1790 state constitution. On December 21, 1790, Mifflin became the last president of Pennsylvania and the first governor of the Commonwealth. He held the latter office until December 17, 1799, when he was succeeded by Thomas McKean. Mifflin died in Lancaster, Pennsylvania, on January 20, 1800. He is interred at Holy Trinity Lutheran Church in Lancaster. A Commonwealth of Pennsylvania historical marker outside Holy Trinity, dedicated in 1975, commemorates Thomas Wharton and Mifflin, the first and last Presidents of Pennsylvania under the Pennsylvania Constitution of 1776.