Mr. Kasit joined the Ministry of Foreign Affairs of Thailand after he graduated from Georgetown University in 1968 and began his career as Third Secretary in the Department of International Organizations. His career path in the Ministry of Foreign Affairs saw him as a leader, with responsibilities such as the Director of the Policy and Planning Division, Deputy Director- General of Economic affairs Department , Ambassador (responsible for European Affairs) and Director- General of the International Organization Department.
His overseas postings included the Royal Thai Embassy in Brussels and the Thai Mission to the European Community [European Union] between 1975 to 1979. Prior to his retirement from the diplomatic career, he was Ambassador to Moscow (in 1991); Jakarta; Bonn/Berlin; Tokyo and Washington D.C., respectively. After retiring from the diplomatic force, he entered into politics and was Minister of Foreign Affairs of Thailand during December 2008 – August 2011 under the Democrat Party-led government. He is currently a member of the National Reform Steering Assembly set up by the military government. He is also a guest lecturer at various academic institutions, a columnist of the Naewna daily newspaper, and occasional contributor to various publications.