The reign of Mongkut was marked by significant modernization initiatives and diplomatic engagements, which played pivotal roles in shaping Thailand's trajectory towards progress and international relations.

By tradition, Mongkut should have been crowned the next king, but the nobility instead chose the older, more influential and experienced Prince Chetsadabodin (Nangklao), son of a royal concubine rather than a queen.

A strong theme in Mongkut's movement was that, "…true Buddhism was supposed to refrain from worldly matters and confine itself to spiritual and moral affairs".

"[7] King Mongkut would later be noted for his excellent command of English, although it is said that his younger brother, Viceroy Pinklao, could speak it even better.

Mongkut's first son and heir, Chulalongkorn, granted the Thammayut sect royal recognition in 1902 through the Ecclesiastical Polity Act; it became one of the two major Buddhist denominations in modern Thailand.

Chulalongkorn also persuaded his father's 47th child, Vajirañana, to enter the order and he rose to become the 10th Supreme Patriarch of Thailand from 1910 to 1921.

Prince Mongkut was supported by the pro-British Dit Bunnag who was the Samuha Kalahom, or Armed Force Department's president, and the most powerful noble during the reign of Rama III.

He also had the support of British merchants who feared the growing anti-Western sentiment of the previous monarch and saw the 'prince monk' Mongkut as the 'champion' of European influence among the royal elite.

Bunnag, with the supporting promise of British agents, sent his men to the leaving-from-monk-status ceremony for Prince Mongkut even before Nangklao's death.

He ordered the nobility to wear shirts while attending his court; this was to show that Siam was a "modern" nation from the Western point of view.

[10] The Traiphum, which was a geo-astrological map created before the arrival of Westerners, described "…a path between two mountain ranges through which the stars, planets, moon and sun pass.

"[10] Religious scholars usually concluded that Buddhist scriptures "…were meant to be taken literally only when it came to matters of spiritual truth; details of natural science are revealed figuratively and allegorically.

[13] However, geography was only taught in select schools, mainly those that were run by American missionaries with English programs for upper secondary students.

[14] Thongchai Winichakul argues that Mongkut's efforts to popularize Western geography helped bring reform to education in Siam.

Previously, Siamese nobles were forbidden to wear any shirts to prevent them from hiding any weapons in it and met the king bare-chested.

He also began the Magha Puja (มาฆบูชา) festival in the full moon of the third lunar month, to celebrate Buddha's announcement of his main principles.

[7] The main principle of the treaty was to abolish the Royal Storage (พระคลังสินค้า), which since Ayutthaya's times held the monopoly on foreign trade.

However, this led to dramatic growth of commercial sectors as common people gained access to foreign trade.

People rushed to acquire vast, previously empty fields to grow rice and the competition eventually resulted in the lands ending up in the hands of nobility.

Mongkut's reign saw immense commercial activities in Siam for the first time, which led to the introduction of coinage in 1860.

Infrastructure was improved; there was a great deal of paving of roads and canal digging—for transport and water reservoirs for plantations.

In 1862, following a recommendation by Tan Kim Ching in Singapore, the court hired an English woman named Anna Leonowens, whose influence was later the subject of great Thai controversy.

To clarify the historical record, well-known Thai intellectuals Seni and Kukrit Pramoj in 1948 wrote The King of Siam Speaks.

The Pramoj brothers sent their manuscript to the American politician and diplomat Abbot Low Moffat[15] (1901–1996), who drew on it for his 1961 biography, Mongkut the King of Siam.

[16] Anna claimed that her conversations with Prince Chulalongkorn about human freedom, and her relating to him the story of Uncle Tom's Cabin, became the inspiration for his abolition of slavery almost 40 years later.

[18] During his monkhood, Mongkut studied both indigenous astrology and English texts on Western astronomy and mathematics, hence developing his skills in astronomical measurement.

[24] Upon returning from his journey to Wakor, he condemned the court astrologers "for their...stupid statements because of their negligence of his detailed prediction and their inattention to measurement and calculation by modern instruments.

He did, however, offer to send some domesticated elephants to US president James Buchanan, to use as beasts of burden and means of transportation.

[29][30] A century later, during his state visit to the US, King Bhumibol Adulyadej of Thailand, who was Mongkut's great-grandson, referred to this event in his address before the US Congress on 29 June 1960.

He said, "my great-grandfather offered to send the President and Congress elephants to be turned loose in the uncultivated land of America for breeding purposes.