He was the former Secretary of Foreign Affairs of the Republic of the Philippines and Director-General of the United Nations Industrial Development Organization (UNIDO) from 1985 to 1992.

[2] Siazon obtained his Bachelor of Arts degree in political science from the Ateneo de Manila University.

He earned a second degree in Physics from the Tokyo University of Education, where he studied as a Japanese government scholar.

[3] He was fluent in Ilocano, English, French, Spanish, Japanese, German, and Tagalog.

As Foreign Secretary, he oversaw the establishment of formal diplomatic ties between the Philippines and North Korea in July 2000.