Chulalongkorn was the ninth son of King Mongkut, but since he was the first to be born to a royal queen, he was recognized as heir to the throne. He was only 15 years old when his father died in October 1868, and he succeeded to the throne under the regency of Somdet Chao Phraya Si Suriyawong (Si Suriyawong, Somdet Chao Phraya) (q.v.). Over the next five years he was prepared to assume his duties by observing court business and by travels to British Malaya and the Dutch East Indies in 1871 and to Malaya, Burma (Myanmar), and India in 1871–72.

Following his coronation in November 1873, the young king enacted a series of ambitious reforms, beginning with the abolition of slavery, the improvement of judicial and financial institutions, and the institution of appointed legislative councils. His commitment to reforms patterned on Western models, which he considered vital to Siam's survival, antagonized conservative factions at court and precipitated a political crisis early in 1875. Rebuffed by the older generation, the king instituted no further reforms for the next decade, but he slowly built up a corps of able, trusted administrators with whom he began from the mid-1880s to overhaul Siam's antiquated administration. These steps culminated in 1892 with the creation of 12 ministries functionally organized on Western lines, responsible for such functions as provincial administration, defense, foreign affairs, justice, education, and public works. He thereby drastically curbed arbitrary administration, ended the autonomy of outlying provinces, instituted the rule of impersonal law, and laid the foundations of modern Thai citizenship through compulsory primary education and universal military conscription.

Internal reforms were undertaken both because the liberal king believed them to be right and because he recognized that he had to show the colonial powers that Siam was “civilized” in order to avoid the fate of neighbouring countries that fell under colonial rule. Even so, old Siam did not survive intact. The French provoked war with Siam in 1892, and by treaties with France up to 1907 Siam had to give up its rights in Laos and western Cambodia. In 1909 Siam ceded to Great Britain the four Malay states of Kelantan, Trengganu, Kedah, and Perlis, and this brought some moderation of the system of extraterritoriality—which ended only two decades later. In relations with the West, Chulalongkorn even-handedly balanced the colonial powers against one another and consistently sought to have Siam treated as an equal among nations. During tours of Europe in 1897 and 1907, he was received as an equal by Western monarchs. When Chulalongkorn died in 1910, following the longest reign in Thai history, he bequeathed to his son Vajiravudh a modern, independent kingdom.