I'm no specialist in pre-constitutional US by any stretch, but if I'm reading the article on the Articles of Confederation correctly (emphasis mine):
On March 4, 1789, the government under the Articles was replaced with the federal government under the Constitution. The new Constitution provided for a much stronger federal government by establishing a chief executive (the President), courts, and taxing powers.
In other words the first POTUS is indeed George Washington.
(The President of the Continental Congress' role would have been closer to that of today's House speaker if I am not mistaking - if even that.)