Model UN began as a series of student-led Model League of Nations simulations. It is believed that the first Model League of Nations conferences were held in the 1920s, before transitioning to Model UN after the formation of the League's successor organization, the United Nations, in 1945. Today, some Model UN conferences include simulations of the League of Nations among their committee offerings.

It is disputed which conference was the first Model UN conference held in the world, with the major claimants being Berkeley Model United Nations (BMUN), Harvard Model United Nations (HMUN), and National Model United Nations (NMUN NY). However, it is clear that the first was held some time in the early 1950s.

In recent decades, Model UN has spread to East and South Asia, the Middle East and North Africa, and sub-Saharan Africa, with major conference organizers such as Indian International Model United Nations, Harvard and THIMUN establishing additional conferences in these regions to meet burgeoning demand. The Ivy League Model United Nations, an arm of the Model UN at the University of Pennsylvania, hosts conferences for high-school-aged delegates in India and China, as well. In India and its neighboring countries, Indian International Model United Nations has become the leading MUN Organization by conducting more than 100 conferences.