Jefferson was ambassador to France from the United States, as constituted under the Articles of Confederation. The confusion might stem from the fact that you're conflating the adoption of the current US constitution with the formation of the nation -- although American independence was declared in 1776, the Articles of Confederation were adopted in 1781 and Britain formally recognized American independence in 1783, after conceding defeat in the American revolutionary war. The current Constitution was ratified in 1788 and active in 1789.

Jefferson's tenure as Minister to France lasted from 1784 to 1789, when he was named the first Secretary of State in the Washington administration.