Did you know that Philadelphia, sixth-most populous city in the United States, with an estimated population of 1,567,872, was once the capital of the United States? Yes, you heard that right: good ol’ Philly, which was founded by William Penn, an English Quaker, in 1682 to serve as capital of the Pennsylvania Colony, also served as the temporary capital while the new capital was under construction in the District of Columbia from 1790 to 1800. The mansion at 6th & Market Streets served as the presidential mansion of George Washington and John Adams, 1790–1800