The following materials are required to obtain Mongolian Visa:
Obtaining a Visa for Mongolia is a relatively painless and quick process. Visas are available for various periods of time but as a tourist a 60 day single entry visa is the norm, any longer would require an invitation letter and a clear statement of purpose concerning your stay in Mongolia. For a 60 day single entry visa an invitation letter is not normally necessary. Should an invitation letter be required, most travel agencies will provide you with a confirmation of your bookings with them which should do the trick.
Mongolia has a number of embassies and consulates abroad which will issue visas previous to travel, please check with your nearest embassy as conditions and rates will vary from one to the other. You can often get these in a one day service that most embassies provide. If not it usually takes up to two weeks to process.
Remember, American's don't need to apply for a visa to Mongolia.