His death was confirmed by the government this morning. Tonga's parliament has been adjourned indefinitely, and Mr Pohiva's deputy, Semisi Sika, is the acting prime minister.
Mr Pohiva, who became prime minister in 2014, was flown to an Auckland hospital with pneumonia on Wednesday, his condition steadily deteriorating. Mr Pohiva had been in poor health for the past few years, and he spent much of this year in Auckland being treated for liver complications.
A constant thorn in the side of the monarchy and nobility, Mr Pohiva was for decades the face and voice of Tonga's pro-democracy movement, his lifelong battle seeing him fired from the public service, charged with sedition, and jailed at various points in his storied life.
But he maintained strong support among the people, who repeatedly voted him into the Tongan parliament after he was first elected in 1987, making him the longest-serving people's representative.
In 1992, Mr Pohiva organised a democracy conference. Radio Tonga was banned from making any mention of the event, and speakers invited from overseas were turned away at the border, on government orders. Still, about 700 people attended.
"Those people at the top outcast me, they have been trying to isolate me from the rest of my colleagues and from the people, but they could not," Mr Pohiva told RNZ at the time. "More and more senior people in the country have come to back up and to support the movement."
"I'm very confident that change will happen."
It did. Constitutional changes brought in after the 2006 riots in Nuku'alofa eventually cleared the path for Mr Pohiva to become prime minister in 2014, and he was re-elected in 2017 with an increased majority, despite King Tupou VI dissolving parliament and calling a snap election.
"He was a beacon of democracy in the Pacific," said Malakai Koloamatangi, a Tongan political scientist at Massey University. "He was kind of a reference point for people who wanted more democracy.
"It's a sad day," he said.
Samiuela 'Akilisi Pohiva was born in Fakakai, on the island of Ha'apai, on 7 April 1941. He studied at the University of the South Pacific before beginning a career as a school teacher.
He returned to the USP, teaching history and sociology at the university's campus on Tongtapu, and it was there that he first became involved in the pro-democracy movement.
Under Tonga's 1875 constitution, it was the monarch who wielded most of the country's power, along with a hereditary nobility representing the various village and island groups.
The Privy Council and most of the legislature were appointed by the King.
Then, Tonga's parliament was split between the nobles and the people's representatives. There were nine seats reserved for Tonga's 33 nobles, and nine seats reserved for the people's representatives, who were elected by about 90,000 people.
In the latter half of the 20th century, a democracy movement started simmering and grew to be increasingly outspoken, and at its forefront was 'Akilisi Pohiva.
Then a civil servant, Mr Pohiva started contributing to the movement's radio programme, Matalafo Laukai, and became the assistant editor of its monthly newsletter, Kele'a. As his criticism of the monarchy grew ever more trenchant, Mr Pohiva was fired from the civil service as punishment. He sued for unfair dismissal and won.
He then ran for parliament in 1987 as one of the people's representatives. But even within the system, he continued to push for dramatic change, his political career marked by constant battles with the government, the nobles and the monarchy.
"The existing system is very much like a dictatorship type of system," Mr Pohiva told RNZ at the time.
"People have no voice in the present government and we need more representatives of the people to participate in the running of the government."
Mr Pohiva was often labelled a rebel, but to many Tongans, he was a hero.
Throughout the 1990s, he would spend much of his time in and out of the courts, charged with all manner of things from libel to sedition.
In 1996, he was imprisoned for contempt of parliament after he was found guilty of leaking information to a pro-democracy newspaper edited by Kalafi Moala, who was in jail with him.
"He was without question the best opposition leader Tonga ever had," Mr Moala said.
He recalled that of the 26 days they spent behind bars, Mr Pohiva spent much of that in hospital suffering from complications with asthma. The trio were released after the Supreme Court ruled their imprisonment unlawful.
"He came into a period in Tongan politics in which changes needed to take place," Mr Moala said. "He was the one that brought in questions, challenges, he was able to bring scrutiny to the governing powers of the day for over 30 years."
In 2005, the country was hit by major strikes and public protests which culminated in the riots that destroyed large parts of the capital, Nuku'alofa, in 2006.
In the wake of those riots, Mr Pohiva was again arrested and charged with sedition, the democracy movement accused by the government of fomenting the unrest.
"What happened ... is the culmination of several social, economic, political and moral forces which is part of the human struggle for their fundamental rights and their fundamental freedoms," Mr Pohiva told RNZ as he was awaiting his sedition trial.
"If we go deeper and analyse the causes of the riot we may come out with a different solution to the problem."
"It would be better for the government to make peace with the people," he said.
It was thought the democratic movement had been set back when the government invoked emergency powers following the riots, but in 2008, King Tupou V announced that elections would be held in 2010, preceded by constitutional reform.
For the first time, the majority of the legislature would be elected by the people, a move which was hailed as a massive victory for the democracy movement.
Dr Koloamatangi said throughout his career, Mr Pohiva became the most prosecuted person in the history of Tonga.
"One thing we cannot take away from Pohiva is the fact that I've never seen anyone persevere for that long for a single cause," he said.
Mr Pohiva contested the 2010 elections under the banner of his new political party, the Democratic Party of the Friendly Islands, which he formed with members of the kingdom's human rights and democracy movement.
A noble, Lord Tu'ivakano, was narrowly elected prime minister by a vote in parliament, but Mr Pohiva would accept a position in the cabinet, becoming health minister.
That only lasted a few months. He soon resigned in protest, becoming the de facto opposition leader.
Then, in the 2014 elections, Mr Pohiva finally became the first democratically-elected prime minister in the country's second democratically-elected parliament.
However, his government became marred by controversy and complaints of ineffectiveness.
Among the problems faced included changes to the education system, a decision to develop a wetland on Tongatapu, and the dismissal of three cabinet ministers, including one who was convicted of bribery.
"He did not prove to be the prime minister that he had hoped to be," Dr Koloamatangi said.
"He kept on being a political activist and a rebel even though he was prime minister. There was nothing tangible to fight against because he was the establishment. He was maybe fighting against himself."
Part of those problems were attributed to challenges posed by Tonga's nascent democracy, including the considerable power that was still wielded by the throne.
That power was wielded in September 2017 when the King made the sudden decision to dissolve parliament more than a year before an election was due.
The Speaker, Lord Tu'ivakano, had gone to the palace to vent his frustrations with Mr Pohiva, and asked for the dissolution.
But Mr Pohiva's support remained strong. In the November 2017 elections, the prime minister was returned with an increased majority.
His final official trip abroad was to Tuvalu in August, for the Pacific Islands Forum leaders' meeting.
There, he was frail and barely able to walk. But he gave some of the most impassioned moments.
At the leaders' meeting he broke down in tears speaking of inaction on climate change; at a meeting with civil society leaders, he railed against the region's decades of inaction on West Papua.
But prophetically, he said it would probably be his last trip.