The U.S. Constitution was written by the delegates to the Philadelphia Constitutional Convention of 1787. The Constitution combined inputs from many people as well as many documents such as the Articles of Confederation and the Declaration of Independence.
Many of the delegates to the Constitutional Convention were also the country's founding fathers: George Washington who presided over the convention, James Madison who wrote the document, and Benjamin Franklin who was the oldest delegate. Other founding fathers who provided important input are Thomas Jefferson who wrote the Declaration of Independence, John Adams who wrote the Defense of the Constitution of the Government of the United States of America and Patrick Henry who supported the inclusion of the Bill of Rights.
The Constitution was ratified by 11 of the original states in 1778 and by all 13 original states by 1790. The original copy of the Constitution can be found in the National Archives in Washington D.C.