In 1924, he was promoted to the rank of under-secretary for foreign affairs, and was responsible for negotiating several important amendments to political and commercial treaties with Western powers.

During that period, he also served as head of the Thai delegation to the League of Nations, where he was active in a number of important commissions as member, vice-president, and president.

Prince Wan returned to Thailand in 1930, to accept a professorial chair at the Faculty of Arts, Chulalongkorn University.

For the next 30 years, Prince Wan continued to serve his country in a number of important diplomatic missions, some of the notable milestones being negotiations with Japan in 1943 during World War II, representing Thailand at the Greater East Asia Conference, participation in the SEATO Council and the Bandung Conference, where he was elected rapporteur, and negotiations leading to Thailand's admission to the United Nations.

[1] Prince Wan won many academic honours and is regarded as one of the founding fathers of philological textual criticism in Thailand.