At its inception, The Colombo Plan was a hallmark Commonwealth initiative with all seven founding members being realms of the British Empire. India, Pakistan, and Ceylon (now Sri Lanka) went on to become Republics in 1950, 1956 and 1972 respectively. The United States of America was the first non-Commonwealth country to join in 1951.

Today, the Colombo Plan boasts a membership of 28 countries, with Chile being the last new member to join in 2021.

The following is a list of all member countries, in alphabetical order (or by region) and their respective year of accession to the Colombo Plan or its predecessor, the Colombo Plan Bureau for Technical Co-operation.