The United States Constitution took effect under the new federal government in 1789.
The Constitution was drafted at the Philadelphia Convention in 1787 and signed on September 17 of that year. It became legally effective after New Hampshire supplied the ninth ratification on June 21, 1788, but the new government began operating in 1789.
Congress convened under the Constitution on March 4, 1789, and George Washington was inaugurated as president on April 30. The Constitution replaced the Articles of Confederation, which had created a weaker national government with no separate executive branch.
The Bill of Rights was proposed in 1789 and ratified in 1791, so that later date is a common distractor. The Constitution remains the oldest written national constitution still in use, though it has been amended 27 times.