In contrast with the four adjoining Federated Malay States of Selangor, Perak, Pahang, and Negeri Sembilan, the five Unfederated Malay States lacked common institutions, and did not form a single state in international law; they were in fact standalone British protectorates.

Penang and Malacca, which had formed a part of the Straits Settlements, were then grouped with the Unfederated Malay States and the Federated Malay States to form the Malayan Union.

The Federation of Malaya gained full independence from the United Kingdom on 31 August 1957.

Under the Bangkok Treaty of 1909, Siam transferred its rights over Kelantan, Terengganu, Kedah, and Perlis to the United Kingdom.

With the assistance of Japan, they temporarily returned to Thai jurisdiction for the latter part of World War II.