Arthur St. Clair was the President of the United States in Congress Assembled from February 2, 1787 to November 4, 1787, and Cyrus Griffin held that office from January 22, 1788 to November 2, 1788. I suppose that Congress was not in session November 5, 1787 to January 21, 1788 and November 3, 1788 to George Washington's inauguration April 30, 1789. The manner of running the country in the absence of a President seems most unclear.
Before George Washington there were many Presidents of the Continental Congress and of the United States in Congress Assembled:
(Presidents of the First Continental Congress)
(Presidents of the Second Continental Congress)
Richard Henry Lee
Arthur St. Clair
(Presidents of the United States in Congress Assembled)