The French ambassador in a foreign country (or the permanent representation to an international organisation) is appointed by the President of the Republic in a meeting of the Council of Ministers on the suggestion of the Minister of Foreign Affairs, that is to say following a very formal procedure. It is one of "the high ranking positions filled by decision of the government", which is therefore able in law to appoint whoever it chooses.
There is such a procedure in the United States, where Ambassadors change with the President. In France, it is rare. The choice is nearly always a diplomat with a good mastery of the profession, in other words,diplomats that have reached the grade of envoy or at the very least the grade of counsellor (first class).
A "marshal’s baton": in principle ambassadors bear this title for the duration of their appointment. For some the crowning achievement of their career is, according to the established formula, being "promoted to the dignity of the Ambassadors of France" meaning they will retain this title for life.
Once the future ambassador has been designated by the Council of Ministers, he has to obtain the approval of the country to which he will be going. It is only then that the appointment can be announced and published. Once he has arrived at his posting, he is received by the Minister of Foreign Affairs and subsequently by the Head of State, to whom he gives a letter from the President of the Republic which is notice of his appointment: this letter is known as the "credentials" (always in the plural). He is then accredited and enjoys the immunities and privileges guaranteed under international law which are designed to protect the ambassador from any pressure from the authorities of the host country.
The ambassador assumes the complete powers of the state in his posting. He is responsible for all that is of interest to France in the country to which he has been posted. The Ministry makes every effort to guide him in exercising this responsibility.