He was born in the United States, where his family was living while his father was participating in the Public Health program at Harvard University. Less than a year later, in 1928, the family returned to Thailand.
He began school soon there after, where he briefly attend Mater Dei School. In 1933, his mother moved the family to Switzerland, where he continued his education at L'Ecole Nouvelle de la Suisse Romande. He obtained his high school diploma in French Lit, Latin & Greek from the Gymnase Classique Cantonal of Lausanne. In 1945, he began studying science at the University of Lausanne before the family returned to Thailand after the end of WW II.
He ascended the throne following the death of his brother from a gunshot wound on 9 June 1946, at the age of 18 1/2. Because he was so young, his uncle was appointed Prince Regent for him, so he could return to Switzerland to finish his education in law and political science. While still in Switzerland, the young King authorized a military coup the overthrew the government of Luang Thamrong Navasawat and ushered in 30+ years of uninterrupted military dictatorship. He also was responsible for the authorization of the 1949 Constitution which gave the monarchy back almost all of it's powers that were taken when the revolution happened in 1932.
He was a frequent Paris visitor while he was finishing his degree, it was in Paris where he first met the Thai ambassador to France's daughter, Mom Rajawongse Sirikit Kitiyakara. On 4 October 1948, the King was in an automobile accident in Switzerland in which, he lost the sight in his right eye among other injuries. Mom Rajawongse Sirikit Kitiyakara was near him during his recovery, and even transfer to a school nearby to be closer to him. In 1949, an engagement was quietly announced, and they were married on 28 April 1950, a week before the King's official corrination.
Together, they have four children and several grandchildren:
- Princess Ubol Ratana (1951, born in Switzerland)
- HRH The Crown Prince Maha Vajurakibgjirn (1952)
- HRH The Princess Maha Chakri Sirindhorn (1955)
- HRH The Princess Chulabhorn Walailak (1957)
- HRH Princess Bajrakitiyabha (1978, The Crown Prince)
- HSH Prince Juthavachara Mahidol(1979, The Crown Prince)
- HSH Prince Vacharaesorn Mahidol ( 1981, The Crown Prince)
- HSH Prince Chakriwat Mahidol (1983, The Crown Prince)
- Khun Ploypailin Jensen (1981, Princess Ubol Ratana)
- HRH Princess Siribhachudhabhorn (1982, Princess Chulabhorn)
- Khun Bhumi Jensen (August 16, 1983 – December 26, 2004 Princess Ubol Ratana)
- HRH Princess Adityadhornkitikhun(1984, Princess Chulabhorn)
- Khun Sirikitiya Jensen (1985, Princess Ubol Ratana)
- HSH Prince Vacharaesorn Mahidol,(1985, The Crown Prince)
- HRH Princess Sirivannavari Nariratana (1987, The Crown Prince)
- HRH Prince Dipangkorn Rasmijoti (2005, The Crown Prince)