In 1967, the Department of Forestry surveyed and explored the Than Mayom waterfall area on Koh Chang. This area was known as the site of royal visits to the island by King Rama IV and Rama V.
A few years later, in 1973, the Department of Forestry approved in principle the establishment of Than Mayom Waterfall Forest Park.
The following year, 1974, the forest park officially opened. The first officer in charge of the park was Mr. Thanong Hotpavanont.
On 28 July 1981, the National Park Board passed a resolution to establish a marine national park in the area of Koh Chang.
The Royal Forest Department dispatched Mr. Ruangsilp Prakonsri, a well known conservationist, to conduct a survey to find detailed information. He concluded that Koh Chang and its satellite islands, in general, have beautiful scenery with waterfalls and a wide diversity of flora and fauna. In addition, it was noted that Koh Chang also had a significant history in World War Two. When it was the site of the naval battle of Koh Chang, between Thai and French forces.
At a meeting on 15 June 1981, the Department of Forestry presented the findings to the National Park Board. A resolution was past stating that the Koh Chang Islands should be established as a national park.
The royal decree which signed the Mu Ko Chang National Park into force was issued on 31 December 1981. The park covered a total area of 650sqkm. Of which 192 sqkm was land on Koh Chang and smaller islands and 458sqm was sea.