Sihamoni was born in 1953. At the time of his birth and that of his younger brother, his mother Monique Izzi, a Cambodian citizen of French-Corsican-Italian and Khmer ancestry, had been one of King Norodom Sihanouk's consorts after being a constant companion since the day they met in 1951, when she won first prize in a national beauty contest. She was granted the title of Neak Moneang and the name of Monineath at the time of her marriage to King Norodom Sihanouk in 1952. Furthermore, Queen Monineath is a step-granddaughter of the late Prince Norodom Duongchak of Cambodia, and the daughter of Pomme Peang and of her second husband, Jean-François Izzi, a French-Italian banker. The Royal Ark website entry about the genealogy of the Cambodian royal family states that Sihanouk and Monineath were married twice, once on 12 April 1952, when she was 15, and again ("more formally", according to the website) on 5 March 1955. She is described as Sihanouk's seventh wife.
Norodom Sihamoni has 14 half-brothers and half-sisters by his father; his only full sibling, a younger brother, Samdech Norodom Narindrapong, was born in 1954 and died in 2003.
He has spent most of his life outside Cambodia. As a child Sihamoni was sent to Prague, in then Czechoslovakia, by his father in 1962. During the 1970 coup d'état by Lon Nol, Sihamoni remained in Czechoslovakia, where he attended all three levels of education - elementary school, high school and AMU - Academy of Performing Arts in Prague, and studied classical dance and music almost continuously until 1975, when he graduated. As he spent almost his entire childhood and youth in Prague, the king is fluent in Czech language. A movie directed by Vladimir Sís was shot about the prince in Prague in 1967, under the name "The Other Little Prince (Jiný malý princ).
After graduation in 1975 he left Prague and began to study filmmaking in North Korea, and in 1977 returned to Cambodia. Immediately, the ruling Khmer Rouge government turned against the monarchy, and put the royal family including Sihamoni under house arrest until the 1979 Vietnamese invasion.
In 1981 he moved to France to teach ballet, and was later president of the Khmer Dance Association. He lived in France for nearly 20 years, regularly visiting Prague, where he had spent his childhood and youth.
In 1993 the prince was appointed Cambodia's delegate to UNESCO in Paris, where he became known for his hard work and his devotion to Cambodian culture. He had previously refused an appointment as Cambodia's ambassador to France.
Besides his native Khmer, Norodom Sihamoni is the only ruling monarch who speaks Czech fluently. The king is also fluent in French and is a good speaker of English and Russian.
On 14 October 2004 he was selected by a special nine-member council, part of a selection process that was quickly put in place after the surprise abdication of King Norodom Sihanouk a week before. Sihamoni's selection was endorsed by Prime Minister Hun Sen and National Assembly Speaker Prince Norodom Ranariddh (the new king's half brother), both members of the privy council. He was inaugurated and formally appointed as King on 29 October 2004. King Sihamoni and his parents, King Father Norodom Sihanouk and Queen Mother Norodom Monineath specifically requested that the ceremonies not be lavish because they did not wish the impoverished country to spend too much money on the event. On 29 October 2014, there were celebrations to mark the 10th anniversary of his coronation.
Sihamoni remains a bachelor and has no children. His father Norodom Sihanouk stated that Sihamoni "loves women as his sisters".
Before he was crowned king, his royal title was: Sdech Krom Khun (Khmer: ស្តេចក្រុមឃុន), equating him to the rank of "Great Prince". As king, his title is: Preah Karuna Preah Bat Sâmdach Preah Bâromneath Norodom Sihamoni Saman Bhumichat Sasana Rakkhata Khattiya Khmeraratrat Putthintra Mohaksat Khemareacheana Samuhobhas Kampuchea Ekareacharath Bureanasanti Subheamagala Sirivibunla Khmera Sri Bireat Preah Chao Krung Kampuchea Dhibodi (in romanized Khmer); Khmer: ព្រះករុណាព្រះបាទសម្តេចព្រះបរមនាថ នរោត្តម សីហមុនី សមានភូមិជាតិសាសនា រក្ខតខត្តិយា ខេមរារដ្ឋរាស្ត្រ ពុទ្ធិន្ទ្រាធរាមមហាក្សត្រ ខេមរាជនា សមូហោភាស កម្ពុជឯករាជរដ្ឋបូរណសន្តិ សុភមង្គលា សិរីវិបុលា ខេមរាស្រីពិរាស្ត្រ ព្រះចៅក្រុងកម្ពុជាធិបតី roughly translating to: "His Merciful Excellent Majesty Protector, King Norodom Sihamoni, who united the nation, religion, realm, and people of Khmer state, great king who is supported by Buddha and Indra, protector of independence, unification, and peace, King of Cambodia, the Great King in the Kingdom of Cambodia". His given name, Sihamoni, comprises two morphemes from his parent's given names, Sihanouk and Monineath.
On 12 December 2008, Sihamoni selected twenty-six members of the Cambodian royal family to his advisory court, among them his half-brother Prince Norodom Ranariddh as chief advisor. Other choices included Prince Sisowath Sirirath, Princess Norodom Marie (estranged wife of Prince Ranariddh) and Prince Sisowath Thomico.Member of the High Council of Francophone Countries (2004).
Foreign Associate Member of the Académie des Inscriptions et Belles-Lettres (2008).
Slovakia: Statue of St. John of Nepomuk in Divina, patron of the project of restoration (2017).
Grand Cross of the Royal Order of Cambodia (Cambodia)
Grand Cross of the Royal Order of Monisaraphon (Cambodia)
Grand Officer of the Legion of Honor (France, 2004)
Grand Cross of the Legion of Honor (France, 2010)
Grand Cordon of the Order of the Chrysanthemum (Japan, 18 May 2010)
Honorary Citizen of the City of Prague (Czech Republic, 2006)
Silver Medal of the City of Paris (France)