A member of the Chakri dynasty, which has occupied the Thai throne since the 18th century, US-born and Swiss-educated Bhumibol was a deeply revered figure whose unifying appeal stretched across the spectrum of Thai society, from the rich urban elite to poor provincial farmers.
His seven-decade reign began in the aftermath of Thailand's occupation by Japan during the Second World War, and ended deep into the Internet age.
During the upheavals of the 20th century, he skilfully charted a course that put the monarchy at the center of Thai society, acting as a force for community and tradition even as the country lurched between political crises and military coups.
Toward the end of his reign, as his health declined, the King's presence in public life waned. Before his death, analysts expressed concern that his passing will remove a vital point of unity in an increasingly divided country.
From 1946, when he acceded to the throne, to the present, Bhumibol reigned over more than 20 prime ministers, more than a dozen coups (both attempted and successful) and repeated constitutional changes. He also helped the country navigate the disruptive effects of the nearby Vietnam War during the 1960s and '70s.
Over the years, the King intervened periodically in political crises, using his influence to try to defuse situations that threatened to destabilize the country. Legally, he had little power, but he carried great sway through his unique position in Thai society.
In 1973, the King helped steer the country out of violent clashes between student demonstrators and military rulers.
His most noteworthy appearance on the political stage came in May 1992, amid violence between pro-democracy campaigners and the army. The King summoned the two leaders of the rival camps; televised images showed both men submissively kneeling before their monarch and helped calm tensions and stop the violence.
The intervention is also considered to have further bolstered the King's moral authority.
Also known as Rama IX -- a reference to his lineage stretching from Rama I, the founder of the Chakri dynasty -- Bhumibol commanded great love and respect within Thailand. An energetic public relations machine promoted his popularity, which led to his portrait being adorned with marigolds from the marble halls of Bangkok office lobbies to the poorest of rural homes.
Criticism against the law
The King represented the country in his travels abroad, meeting with foreign leaders. Over the decades, he worked to improve the lives of ordinary Thais through civic programs, drawing on his engineering background to advise on projects like new irrigation systems.
Strict laws, against criticizing the monarch restricted frank discussion of his life and personality. The lese majeste laws carry punishments of up to 15 years in prison, and ordinary Thai citizens, as well as the government, could bring charges on behalf of the King, so the laws were sometimes abused as a political weapon.
Human rights groups have criticized prison sentences handed down to people convicted of defaming or insulting the Thai royal family. Even the King appeared to question the restrictive climate.
"If the King can do no wrong, it is akin to looking down upon him, because the King is not being treated as a human being," he said in his 2005 birthday speech. "The King can do wrong."
But his words seemed to do little to deter the prosecutions. As the political situation in Thailand deteriorated in the wake of a military coup that ousted populist leader Thaksin Shinawatra in 2006, successive administrations pursued more lese majeste cases.
According to Human Rights Watch, between January 2006 and May 2011, more than 400 lese majeste cases were brought to trial. The number of prosecutions decreased after Prime Minister Yingluck Shinawatra, Thaksin's younger sister, took office in 2011, the rights group said.
Yingluck was herself ousted from power in a military coup in May 2014, in the two years that followed, Human Rights Watch said there were 68 lese majeste cases brought to trial, relating to opinions, poems, cartoons and online comments.
Analysts said the laws prevented public discussion in Thailand on what would happen after the King's death, deepening uncertainty and political tensions.
Born in 1927 in Cambridge, Massachusetts, and raised outside Thailand, Prince Bhumibol Adulyadej of Songkla was a keen photographer and jazz saxophonist in his youth. He engaged in a range of pursuits during his life and received four patents for a number of environmental and agricultural projects, including one for an artificial rainmaking technique.
Bhumibol inherited the throne after the death of his brother, King Ananda Mahidol, in 1946. The circumstances surrounding the death of Ananda, who was found in bed with a gunshot wound to his head, remain mysterious.
The new King was not formally crowned until May 1950, becoming the head of state of a constitutional monarchy. The Chakri dynasty had absolute rule before 1932, when a coup established the enduring presence of the military in Thai politics.
For much of the first few decades of Bhumibol's reign, the day-to-day running of the country was dominated by the Thai military and bureaucratic elite. Governments replaced one another through a lengthy sequence of mainly nonviolent coups.
The painful and slow transition toward a more democratic government began in the 1970s, with several setbacks along the way.
Of the many changes Thailand has undergone since Bhumibol came to the throne, the growth in the nation's economy is among the most spectacular. Largely agrarian when he came to power, Thailand has since become an industrial and service-sector giant and is the second largest economy in Southeast Asia after Indonesia.
It remains one of the world's top rice producers and has become one of the most popular tourist destinations. It is also a regional manufacturing hub for the motor vehicle industry and hi-tech electronics.
As wealth has poured into the country, living standards have risen significantly, upsetting the social balance in a country that leading scholars dubbed a "network monarchy": an extensive network of royal patronage that extends outward from the palace into the rest of society.
The largely rural and populous northeast of the country, a region in which Bhumibol worked enthusiastically to promote agricultural and social development, has experienced rising prosperity, creating a growing middle class.
The northeast was also a bastion of support for Thaksin, whose populist policies helped generate a political movement that unsettled the established elites. Since Thaksin was toppled in 2006, Thailand has been riven by political instability, as the country divided into political groupings that drew their support from a complex network of associations: on one side, the generally pro-Thaksin red shirts whose support comes mainly from rural communities in the north and east; on the other, the urban and middle class elites, who largely favour a less populist, more paternalistic form of governance.
Despite sometimes violent factionalism, all sides claim allegiance to the King. His intense popularity allowed for widespread celebrations of his 80th birthday in 2007, when millions wore yellow, a color associated with the King.
But amid the political instability of recent years, Bhumibol's health began to fade and public appearances became more infrequent.
He was admitted to a Bangkok hospital in September 2009 with respiratory problems, a stay that lasted four years. In October 2014, he returned to have his gallbladder removed, staying in the hospital for another seven months.
The political chaos was followed by another military coup. The army chief who led the seizure of power in May 2014 announced shortly afterward that he had received the King's endorsement.
When the King left the hospital in May 2015, he was greeted by crowds of well-wishers as he moved to the summer palace. But just a few weeks later he returned to the hospital and had remained there since, receiving treatment for fluid on the lungs and surgery to widen the arteries in his heart.
Some observers of Thailand, like the historian Charles Keyes of the University of Washington, have said the looming prospect of change in the monarchy has exacerbated political tensions in the country in recent years. With the King's death, a vital source of political legitimacy has perished.
King Bhumibol will be a hard act to follow. His expected successor and designated heir is Crown Prince Maha Vajiralongkorn.
As well as the crown prince and his wife, Queen Sirikit, Bhumibol is survived by three other children: Ubolratana, Chulabhorn and Maha Chakri Sirindhorn.