The Mayflower Compact established that the Pilgrims and other settlers who arrived at Cape Cod aboard the Mayflower in November 1620 would abide by the laws and ordinances of a "civil body politic" they would mutually set up to govern themselves. It was the foundation of Plymouth Colony and the first act of government ever written and implemented in the territory that became the United States.
When the Mayflower left England, its intended destination was Virginia. However, due to inclement weather, they were blown several hundred miles off course. This left them outside the jurisdiction of the Virginia Company and independent of the charter they had signed with it. The 102-person group contained non-Puritan settlers such as Miles Standish and John Alden. To prevent them from separating from the group and establishing their own colonies, the Mayflower Compact was drawn up to ensure unity. All adult males signed the compact before they went ashore. The first act of the colonists after signing was to elect John Carver as governor. The Mayflower Compact remained in effect until the Plymouth Colony became part of the Massachusetts Bay Colony in 1691.
According to an About.com article on American history, the Mayflower Compact was a key document used by the founding fathers in their creation of the Constitution of the United States.