George Washington was first president under the constitution. But not the first president of the United States.

Before him where these, several of whom were Freemasons, under the Continental Congress and Articles of Confederation.

You've come to the right place for the truth.
Washington was President of the United States under the constitution from April 30, 1789 March 4, 1797
The Revolutionary war ended with the treaty of Paris in 1783 recognizing sovereignty of the United States.
The US Constitution began getting signatures in 1789.