With the passing of Queen Elizabeth II, Brunei's Hassanal Bolkiah now tops list with a reign of 54 years.
The 76-year-old Sultan Hassanal Bolkiah ascended the throne of the Muslim country perched on the north of the tropical island of Borneo in 1967.
He comes from a family that has ruled the country for over 600 years and is one of the richest people on the planet.
His decades ruling Brunei have seen the country gain full independence from Britain and living standards soar to among the highest globally.
But his reign of Brunei, which has a population of about 400,000, has also been marked by controversies including the introduction of tough Islamic laws legislating penalties such as severing of limbs and death by stoning.
- Denmark's Queen Margrethe II, 50 years -
Denmark's chain-smoking Queen Margrethe II, Europe's longest-serving monarch after the death of Queen Elizabeth, has been hailed for unifying and modernising the monarchy in her 50 years on the throne.
Now 82, she came to power in 1972 at a time of waning support for the royals, but her popular reign has buoyed its standing.
Widowed since 2018, the sovereign fondly named Daisy by her subjects and family, has avoided scandal and helped modernise the institution, including allowing her two sons to marry commoners.
- Sweden's Carl XVI Gustaf, 48 years -
Less than a year old when his father, the heir to the throne, died in a plane crash, 76-year-old Carl Gustaf XVI ascended the throne aged 27 when his grandfather passed away in 1973.
For many years he struggled to shed his image as a car-mad playboy, having inherited the passion for vintage cars from his father.
The press also teased him for his many gaffes, including misspelling his own name at a 1973 signing. It later emerged that the king suffered from dyslexia, as do two of his children.
Shows of compassion have helped boost his popularity, with one of his finest hours coming in 2004 shortly after more than 500 Swedes lost their lives in the tsunami that swept across tourist resorts in southeast Asia.
- eSwatini: Mswati III, 36 years -
In southern Africa's tiny eSwatini, formerly Swaziland, King Mswati III is the continent's last absolute monarch. He ascended the throne in 1986 at the age of 18.
Mswati, who is now 54, has 15 wives -- his father is said to have had at least 70 -- and the right to choose a new one at the annual Reed Dance. He has more than 25 children.