BANGKOK — For 45 years, Najib Razak was a master of Malaysia’s cutthroat politics. The son and nephew of prime ministers, he was elected to Parliament at 22 and rose to the country’s highest office. As prime minister he was all but untouchable — critics called him the “Man of Steal” — even as $4.5 billion disappeared from a government investment fund he controlled.
On Tuesday, he was finally held to account.
After years of allegations that he pilfered government coffers to lead a life of excess and luxury, he was found guilty on seven corruption counts and sentenced to up to 12 years in prison and fined nearly $50 million. The sentence was stayed pending appeal.
The sweeping verdict was a stunning setback for Mr. Najib, whose political party had recently returned to power after forcing out the reformist government that won elections in 2018.
His political foes applauded the verdict and praised the independence of the trial court. But experts said the verdict could be overturned on appeal in what is likely to be a yearslong process.
The severity of the sentence came as a surprise to some, after the government recently cut a deal with Mr. Najib’s stepson that allowed him to keep millions of dollars that were believed to have been stolen from the same Malaysia Development Berhad fund.
Tuesday’s trial was the first of five related to the theft of billions from the investment fund, commonly known as 1MDB, and Mr. Najib was found guilty on all seven charges of abuse of power, breach of trust and money laundering.
The stolen money was traced by prosecutors to the purchases of a mega-yacht and a Picasso painting, as well as an investment in the Hollywood blockbuster “The Wolf of Wall Street.” About $1 billion of the money ended up in Mr. Najib’s personal bank accounts, creating the national scandal that led to the ouster of his party, the United Malays National Organization, in elections two year ago.
The court rejected Mr. Najib’s defense that the theft was carried out without his knowledge by Jho Low, a wealthy Malaysian businessman who is accused of masterminding the crime and remains an international fugitive.
“After considering all evidence in this trial, I find that the prosecution has successfully proven its case beyond a reasonable doubt,” said Judge Mohamad Nazlan Mohamad Ghazali of the Kuala Lumpur High Court.
Before the sentence was pronounced, Mr. Najib appealed to the judge for leniency, noting his contributions as a public official and that the country prospered during his nine years as prime minister. He said he did not solicit the $9.8 million payment nor was it offered to him.
Mr. Najib, 67, still faces dozens of additional charges. Many of the charges against him carry a sentence of up to 20 years in prison.
Mr. Najib remains a member of Parliament, and his party, popularly known as UMNO, returned to power in February, improving the chances that he will not have to spend time behind bars. His conviction could undermine the credibility of the current government headed by his ally, Prime Minister Muhyiddin Yassin, which came to power without an election and holds a bare majority in Parliament.
James Chin, a professor of Asian studies at the University of Tasmania and an expert on Malaysian politics, said the guilty verdict would likely eventually be overturned on appeal.
“People should not be celebrating, because in the Malaysian context, many of the judgments in these political cases get reversed in the appeals court,” he said. “I expect Najib to appeal, and he will probably win the appeal. In the long term, I expect Najib to get away with it as long as UMNO is in power.”
In this trial, Mr. Najib faced seven charges of money laundering, criminal breach of trust and abuse of power for illegally receiving transfers of $9.8 million from SRC International, a former unit of the investment fund commonly known as 1MDB.
The sentence included 12 years for the abuse of power and 10 years for each of the other charges. In Malaysia, sentences run concurrently, so the maximum he would serve would be 12 years in prison.
Mr. Najib will not lose his seat in Parliament, but he would be prohibited from running for re-election unless his convictions are overturned on appeal.
The court rejected the defense contention that he was a victim of a scam and did not know that money from the government fund had been deposited in his personal account.
The judge also rejected the argument that Mr. Najib received the money as a gift from a Saudi royal and that he was unaware of the balance in his own account.
“It would be extraordinary that the accused as the then-prime minister and finance minister did not know it,” said Judge Nazlan, who presided over the 14-month trial.
In rejecting Mr. Najib’s defense and questioning his credibility, the judge cast doubt on whether the former prime minister’s legal strategy can be effective in his upcoming trials, which will focus on much larger amounts of money that went missing from 1MDB.
The opposition leader of Parliament, Anwar Ibrahim, applauded the verdict and said justice had been served.
“For over a decade the 1MDB scandal has been a blight on our nation’s reputation and has been the source of much anguish for the Malaysian people,” he said. “Money which should have been utilized for development and assisting the poor was diverted to illicit gains benefiting a former prime minister and his friends.”
Mr. Najib’s lead defense lawyer, Muhammad Shafee Abdullah, characterized his client as a victim of his political foes and asked the court for a light sentence because of Mr. Najib’s lifetime in public service. He said Mr. Najib had spent only a tiny fraction of the stolen money on himself, with nearly half going for political purposes and another 14 percent to charity.
After appearing in court, Mr. Najib told supporters gathered outside that he is optimistic he will prevail on appeal.
“This is definitely not the end of the world, because there’s a process of appeal, and we hope that we would be successful there,” he said. “The effort will continue, and to my supporters, I hope they will continue to believe in me, believe in our struggle, and continue to be in positive and high spirits.”
In May, prosecutors dropped money-laundering charges against Mr. Najib’s stepson, the Hollywood producer Riza Aziz, who was accused of receiving $248 million in government funds.
The producer of “The Wolf of Wall Street,” Mr. Riza was allowed to walk away with $83 million under the terms of the agreement with the new attorney general, Idrus Harun.
Last week, the new government and Goldman Sachs reached a $3.9 billion settlement over the company’s role in the scandal. The previous government had sought more than $2.7 billion in fines and had charged more than a dozen executives with fraud. Under the settlement, criminal charges against the bank and the executives were dismissed.
Mahathir Mohamad, 95, who was prime minister from 1981 to 2003, came out of retirement in May 2018 to defeat Mr. Najib and UMNO, the party that Mr. Mahathir once led.
After the election, the authorities raided properties owned by Mr. Najib and his wife, Rosmah Mansour, and seized more than $270 million in cash, jewelry, luxury handbags, tiaras and other valuables. Ms. Rosmah also faces corruption charges.
Mr. Mahathir pledged to pursue justice against Mr. Najib and his cronies, but he lost support within his coalition and stepped down in February.
In the months since, and against the backdrop of the coronavirus pandemic, Mr. Najib’s supporters have returned to power in what opponents have called a “coup” and a “backdoor government.”
Mr. Muhyiddin, who formed a coalition with UMNO, was appointed prime minister without an election and has since refused the opposition’s demands for a vote of confidence in Parliament.
Mr. Najib, whose father and uncle were both prime ministers, is part of Mr. Muhyiddin’s inner circle but does not hold a leadership post.
In cementing control after its return to power, Mr. Muhyiddin’s government has sought to crack down on independent news media, including Al Jazeera, which broadcast a 25-minute documentary earlier this month showing a military-style crackdown on undocumented migrant workers over coronavirus fears.
The authorities announced that they were investigating Al Jazeera for sedition, defamation and violating communications laws. The broadcaster said seven of its staff members have been questioned by the police.