French nationals must have a 30-day tourist visa to enter Mongolia. Therefore, before your departure, you must apply at the Mongolian embassy in Paris up to three months in advance.
You must submit the following to the embassy:
A French passport valid for at least six months after your trip to Mongolia
A recent photo
A duly completed application form
Repatriation insurance
An invitation letter from the receiving entity for visas of 30 days or more
Address of the Mongolian consulate in France:
5 Avenue Robert Schuman, 92100 Boulogne-Billancourt, France