Not only was John Adams not present at the Constitutional Convention in Philadelphia, neither was Thomas Jefferson. Although Jefferson had sent his good friend, James Madison, the Father of the Constitution, trunk-loads of books about government for use in preparing for the Constitution.
Why wasn’t Jefferson there? He was in France, from where he sent those books to Madison. Why was he in France? Because he was sent there by Congress to join Benjamin Franklin and John Adams in preparing the Treaty of Paris which ended the Revolutionary War. Adams went to England and Franklin returned home to Pennsylvania after that treaty was signed.