His Majesty King Mongkut was born on Thursday the 18th of October 1804. He was the second son of Phra Buddha Lertla Naphalai (King Rama II) and Queen Sri Suriyendra, whose first--born son died at birth in 1801. Prince Mongkut was five years old when his father succeeded to the Throne in 1809.
In a letter to an American friend before he himself ascended to the Throne, Prince Mongkut explained his name and title as follows: "Chao" corresponds to the English word "Lord", or the Latin word Dominus. "Fah" means the sky, but when used with a person's name it becomes an adjective of exaltation, equivalent to the phrase "as high as the sky". Mongkut means "crown". The name Chao Fah Mongkut thus means "The High Prince of the Crown," or "His Royal Highness the Crown Prince"
When His Royal Highness Prince Mongkut became King, he was generally known in Siam as "Phra Chom Klao", but foreigners always called him Mongkut.