Who they were
Born in 1804, Mongkut spent the 27 years before his accession as a Buddhist monk. In the monkhood he founded the reformist Thammayut order, devoted to stricter monastic discipline and textual study, and studied Western languages and sciences with missionaries.
What they did
Coming to the throne in 1851, he opened Siam to the treaty system: the Bowring Treaty with Britain (1855), then parallel treaties with other powers. He began modernization — printing, shipbuilding, roads and Western advisers. He employed Anna Leonowens as a governess for his children; her embellished memoirs later fed fictionalized stage and screen portrayals that historians and Thais regard as caricature. A keen astronomer, he computed the total solar eclipse of August 18, 1868 and traveled to Wa Ko to observe it. The prediction proved exact, but he contracted malaria there and died on October 1, 1868.
Legacy
He set the negotiate-and-modernize course that his son Chulalongkorn completed. The Thammayut order remains a major monastic lineage, and for the eclipse he is remembered in Thailand as the father of Thai science.