Visa requirements for Mongolia differ depending on the length of your stay and country of origin. For more information on visa requirements, please visit: http://www.mfa.gov.mn/?page_id=28254&lang=en
UK citizens see - https://www.gov.uk/foreign-travel-advice/mongolia/entry-requirements
US citizens see - http://mongolianembassy.us/visa-information-for-us-citizen-only
US citizens can travel to Mongolia without visa if his/her duration of stay is less than 90 days, in accordance with the Mongolian Visa Policy, effective since July 18, 2001. See list of countries that do not require a short term stay visa to Mongolia.