Before returning to private life in June 1783, Washington called for a strong union. Though he was concerned that he might be criticized for meddling in civil matters, he sent a circular letter to all the states, maintaining that the Articles of Confederation was no more than "a rope of sand" linking the states. He believed the nation was on the verge of "anarchy and confusion", was vulnerable to foreign intervention, and that a national constitution would unify the states under a strong central government. When Shays' Rebellion erupted in Massachusetts on August 29, 1786, over taxation, Washington was further convinced that a national constitution was needed. Some nationalists feared that the new republic had descended into lawlessness, and they met together on September 11, 1786, at Annapolis to ask Congress to revise the Articles of Confederation. One of their biggest efforts, however, was getting Washington to attend. Congress agreed to a Constitutional Convention to be held in Philadelphia in Spring 1787, and each state was to send delegates.

On December 4, 1786, Washington was chosen to lead the Virginia delegation, but he declined on December 21. He had concerns about the legality of the convention and consulted James Madison, Henry Knox, and others. They persuaded him to attend it, however, as his presence might induce reluctant states to send delegates and smooth the way for the ratification process. On March 28, Washington told Governor Edmund Randolph that he would attend the convention but made it clear that he was urged to attend.

Washington arrived in Philadelphia on May 9, 1787, though a quorum was not attained until Friday, May 25. Benjamin Franklin nominated Washington to preside over the convention, and he was unanimously elected to serve as president general. The convention's state-mandated purpose was to revise the Articles of Confederation with "all such alterations and further provisions" required to improve them, and the new government would be established when the resulting document was "duly confirmed by the several states". Governor Edmund Randolph of Virginia introduced Madison's Virginia Plan on May 27, the third day of the convention. It called for an entirely new constitution and a sovereign national government, which Washington highly recommended.

Washington wrote Alexander Hamilton on July 10: "I almost despair of seeing a favorable issue to the proceedings of our convention and do therefore repent having had any agency in the business." Nevertheless, he lent his prestige to the goodwill and work of the other delegates. He unsuccessfully lobbied many to support ratification of the Constitution, such as anti-federalist Patrick Henry; Washington told him "the adoption of it under the present circumstances of the Union is in my opinion desirable" and declared the alternative would be anarchy. Washington and Madison then spent four days at Mount Vernon evaluating the new government's transition.