The Prince undertook military training at the Royal Military College, Duntroon in Canberra, Australia, and also completed an arts degree at Sukhothai Thammatirat University in Bangkok. Since 1975, he has served as a career officer in the Royal Thai Army. He served as a staff officer in the Directorate of Army Intelligence, and in 1978 he became head of the King's Own Bodyguard Battalion. In that year, however, he interrupted his military career to be ordained for a season as a Buddhist monk, as is customary for all Thai Buddhist males. Thai Royal Family HM The King HM The Queen Princess Ubolratana Rajakanya Khun Ploypailin Mahidol Khun Poom Mahidol Khun Sirikitiya Mahidol HRH The Crown Prince HRH Princess Bajrakitiyabha HRH Princess Siriwannawari Nariratana HRH Prince Dipangkorn Rasmijoti HRH Princess Maha Chakri Sirindhorn HRH Princess Chulabhorn Walailak HRH Princess Siribhachudhabhorn HRH Princess Adityadhornkitikhun HRH Princess Bejaratana HRH Princess Galyaniwatana Thanpuying Dhasanawalaya Sornsongkram HRH Princess Srirasmi HRH Princess Soamsavali Vajiralongkorn trained for periods with the United States, British and Australian armed services, studying special forces demolition, unconventional warfare tactics and advanced navigation training. He is a qualified military pilot and a helicopter pilot. Although a military career is conventional for royal princes, Vajiralongkorn is unique in having taken an active part in military actions inside his own country. In the 1970s he led counter-insurgency campaigns against the forces of the Communist Party of Thailand in the North and Northeast of Thailand, and also took part in operations along the border with Cambodia during the years of the Khmer Rouge regime.

Vajiralongkorn now holds the ranks of General in the Royal Thai Army, Admiral in the Royal Thai Navy and Air Chief Marshall in the Royal Thai Air Force. His military role in recent years has become increasingly ceremonial. As his father has grown older, turning 79 in 2006, Vajiralongkorn has taken a more prominent part in royal ceremonial and public appearances. Although the subject is never publicly discussed in Thailand, most observers believe that Vajiralongkorn shares none of the enormous popularity of his parents among the Thai public. This is partly because of his career as a military officer at a time when the public have turned against Thailand's long history of military rule, partly because of his private life, and partly because he is seen as a cold and remote personality. The Economist wrote in 2002: "Vajiralongkorn is held in much less esteem [than the king]. Bangkok gossips like to swap tales of his lurid personal life. One of his sisters, another possible heir to the throne, is more popular, but Thailand has never been ruled by a woman. Besides, no successor, however worthy, can hope to equal the stature King Bhumibol has attained after 55 years on the throne."

On January 3, 1977 Vajiralongkorn married Princess Soamsavali Kitiyakara (born 1957), a first cousin on his mother's side. The marriage ended in divorce in the 1980s, and Vajiralongkorn lived for some years with a common law wife, Yuvadhida Polpraserth (born 1962), with whom he had four sons and a daughter. Although Yuvadhida was made a princess and her children also granted titles, the lack of a legitimate male heir was ostensibly the main reason that Thailand legislated to allow women to inherit the throne. According to the December 1999 issue of Asiaweek, Vajiralongkorn's relationship with Yuvadhida came to an end in 1996 when former prime minister Banharn Silpa-Archa made a statement that the then Air Chief Marshall Anand Rodsamkhan was dismissed from his posting at the Royal Palace, prompting unconfirmed rumors that there had been an extra-marital affair. Yuvadhida and her children fled to England, but Vajiralongkorn travelled to England and brought his daughter back to Bangkok. Vajiralongkorn married again, on February 10, 2001, to Mom Srirasmi Mahidol na Ayuthaya, a commoner from a modest background who was later elevated to HRH Princess Srirasmi. This marriage was not disclosed until recently. She gave birth to a son on April 29, 2005. Vajiralongkorn had children with: With Princess Soamsavali Kitiyakara: HRH Princess Bajrakitiyabha, born December 7, 1978. With Yuvadhida Polpraserth he had five children: HSH Prince Chudhavajra, born August 29, 1979 HSH Prince Vajaresra, May 27, 1981 HSH Prince Chakrivajra, February 26, 1983 HSH Prince Vajravira, June 14, 1985 HSH Princess Busya Nambejira (later changed to Siriwanwari), January 8, 1987 (elevated to HRH Princess Siriwannawari Nariratana by royal command on June 15, 2005). With Princess Srirasmi (elevated to HRH on June 15, 2005) : HRH Prince Dipangkorn Rasmijoti, born April 29, 2005, who apparently becomes second-in-line to the throne after his father.