The Constitutional Convention took place in Philadelphia from May 25 to September 17, 1787. While the convention was initially intended to revise the league of states and the first system of federal government under the Articles of Confederation, leading proponents of the Constitutional Convention, including James Madison of Virginia and Alexander Hamilton of New York, sought to create a new frame of government rather than revise the existing one. Delegates elected George Washington of Virginia, former commanding general of the Continental Army in the American Revolutionary War and a proponent of a stronger national government, to serve as president of the convention. While the Constitutional Convention has been the only Federal one, the fifty states have held 233 constitutional conventions. The convention ultimately debated and ratified the Constitution of the United States, making the convention one of the most significant events in American history.

The convention took place in Pennsylvania State House, later renamed Independence Hall, in Philadelphia. The convention was not referred to as a constitutional convention. It was contemporarily known as the Federal Convention, the Philadelphia Convention, or the Grand Convention at Philadelphia. Most delegates did not arrive intending to draft a new constitution. Many assumed that the convention's purpose was to discuss and draft improvements to the existing Articles of Confederation, and would not have agreed to participate otherwise. Once the convention began, however, most of the delegates – though not all – came to agree in general terms that the goal would be a new system of government, not simply a revised version of the Articles of Confederation.

Several broad outlines were proposed and debated, notably Madison's Virginia Plan and William Paterson's New Jersey Plan. The Virginia Plan was selected as the basis for the new government, and the delegates quickly reached consensus on a general blueprint of a federal government which has three branches (legislative, executive, and judicial) along with the basic role of each branch. However, disagreement over the specific design and powers of the branches delayed progress for weeks and threatened the success of the convention. The most contentious disputes involved the legislature, specifically the composition and election procedures for the Senate as the upper legislative house of a bicameral Congress, and whether proportional representation was to be defined by a state's geography or by its population.

The role of the executive was also hotly debated, including the key issues of whether to divide the executive power among three people or vest the power in a single chief executive to be called the President; how a president would be elected; the length of a presidential term and the number of allowable terms; what offenses should be impeachable; and whether judges should be chosen by the legislature or the executive. Slavery was also a highly contentious issue, with delegates debating the inclusion of a fugitive slave clause, the potential abolition of the slave trade, and whether enslaved individuals should be counted for purposes of proportional representation. A significant portion of the Convention's time was devoted to resolving these matters.

Progress was slow until mid-July, when the Connecticut Compromise resolved enough lingering arguments for a draft written by the Committee of Detail to gain acceptance. Though more modifications and compromises were made over the following weeks, most of this draft can be found in the finished version of the Constitution. After several more issues were debated and resolved, the Committee of Style produced the final version in early September. It was voted on by the delegates, inscribed on parchment by Jacob Shallus with engraving for printing, and signed by 39 of 55 delegates on September 17, 1787. The completed proposed Constitution was printed in several copies for review which began the debates and ratification process. Soon after, it was also printed in newspapers for public review.

During the American Revolution, Thirteen Colonies replaced their colonial governments with republican constitutions based on the principle of separation of powers, organizing government into legislative, executive, and judicial branches. These revolutionary constitutions endorsed legislative supremacy by placing most power in the legislature, which was viewed as most representative of the people, including power traditionally considered as belonging to the executive and judicial branches. State governors lacked significant authority, and state courts and judges were under the control of the legislative branch at the time. Massachusetts popularly elected delegates in 1779 for its constitutional convention, establishing a precedent for the Federal one.

The Second Continental Congress, which convened in present-day Independence Hall in the colonial capital of Philadelphia, adopted the Declaration of Independence in 1776, and the 13 colonial states created a permanent alliance to coordinate American efforts to win the Revolutionary War. This alliance, the United States, was governed according to the Articles of Confederation, which was more of a treaty between independent countries than a national constitution. The Articles were adopted by the Second Continental Congress in 1777 but not ratified by all states until 1781.

Under the Articles, the United States was essentially a federation of independent republics, with the Articles guaranteeing state sovereignty and independence. The Confederation was governed by the Congress of the Confederation, a unicameral legislature whose members were chosen by their state legislatures and where each state was entitled to one vote. Congress's powers were limited to waging war and directing foreign affairs. It could not levy taxes or tariffs, and it could only request money from the states and could not force delinquent states to pay. Since the Articles could only be amended by a unanimous vote of the states, each state had effective veto power over any proposed change. A super majority (nine of thirteen state delegations) was required for Congress to pass major legislation such as declaring war, making treaties, or borrowing money. The Confederation had no executive or judicial branches, which meant the Confederation government lacked effective means to enforce its own laws and treaties against state non-compliance. It soon became evident to nearly all that the Confederation government, as originally organized, was inadequate for managing the various problems confronting the United States.