When you start planning for your trip, the foremost step is to check the validity of your passport. Mongolia requires that remaining validity of your passport must be minimum 6 months when you depart the country. You also need to make sure you are not running out blank visa pages.
US citizens, Canadians, Germans and Japanese are entitled visa free entry to Mongolia for tourism purpose. Some other countries require no visa to enter Mongolia on basis of the passport type, purpose and duration of the visit in accordance with the Bilateral Agreement of Mongolia with those countries for certain duration. Please, check the list if you are the citizen of the countries that can have visa free entry to Mongolia. http://www.mfa.gov.mn/?page_id=18131&lang=en
The duration does vary for all countries as: 90 days, 30 days, 21 days and 14 days. And for all the other citizens of foreign countries not listed in “no visa required” section is required to obtain appropriate visa to visit Mongolia. Normally visa is processed in 2 working days. However, please be advised that the regulations require up to one week for the processing. As for extension process, visitors can extend on the basis of per-week and you may be asked for a flight itinerary printout when requesting an extension. And, if you overstay your visa, you will be fined as per their regulations.