In a huge political upset, former strongman Mahathir's opposition alliance broke the grip on power of the Barisan Nasional (BN) coalition, which had governed Malaysia uninterrupted since its birth as an independent country in 1957.
It capped a dramatic comeback for Mahathir, who previously ruled the country at the head of BN for 22 years and came out of retirement to take on his ex-protege, Prime Minister Najib Razak, after the leader became embroiled in a massive corruption scandal.
Ninety-two-year-old Mahathir Mohamad was sworn in on May 10 as the world's oldest elected leader. Source: AAP
In a ceremony at the national palace steeped in centuries-old Muslim Malay tradition, Mahathir was officially sworn in as prime minister by King Sultan Muhammad V.
Mahathir, wearing traditional Malay dress, took the oath of office in an elaborately decorated room, with political allies and senior government officials in attendance.
As he was inaugurated, hundreds of revellers outside the palace cheered and sang the national anthem, with some breaking down in tears. Fireworks lit up the night sky across Kuala Lumpur.
"We feel so united tonight," 24-year-old student Abdul Aziz Hamzah, who was among the crowd, told AFP. "Mahathir is so insightful and experienced because he's been here before."
One of the most remarkable aspects of Mahathir's comeback has been his reconciliation with jailed opposition icon Anwar Ibrahim, his former nemesis. Their relationship has loomed large over the Malaysian political landscape for decades.
Prime Ministerial candidate Mahathir Mohamad (C-L) poses for a selfie Source: AAP
Anwar was Mahathir's heir-apparent until the premier sacked him in 1998 over political differences, and he was subsequently jailed on charges of sodomy and abuse of power.
Anwar was jailed again during Najib's rule but is due out in June -- and Mahathir has pledged to help him get a royal pardon, allowing him to run for political office again and potentially become prime minister.
'Heads must fall'
The opposition faced an uphill battle at the election due to what critics said were no-holds-barred attempts by Najib to hang on to power.
His government was accused of gerrymandering while activists said he hurled cash and gifts at voters and there was a litany of problems with the electoral roll, including dead people appearing on the list.
Najib called the vote mid-week, in what critics said was an effort to keep turnout down -- elections are normally held at a weekend in Malaysia. But in the end voters flocked to the polls in droves in a determined bid to push him out.
Najib had initially refused to offer a clear concession. But after Mahathir was sworn in, he congratulated him on Twitter and said he was "willing to assist a smooth transfer of power".
At a press conference after being sworn in, Mahathir said the government's main focus would be on the economy. A major concern for Malaysians in recent times has been the soaring cost of living.
He said his administration would abolish laws that are "oppressive", and that widely-condemned legislation outlawing "fake news" would be examined.
The new government would look into clawing back money allegedly looted from sovereign wealth fund 1MDB, he said. Billions of dollars were allegedly stolen from the investment vehicle that was set up and overseen by Najib, in a massive scandal that battered Malaysia's international image.
Both Najib and 1MDB deny any wrongdoing.
Mahathir has previously vowed that Najib will face justice over the controversy.
He also said that "certain heads must fall" in government departments.
"We found that some people were aiding and abetting a prime minister which the world condemns as a kleptocrat," he said.
Initial delight felt by many at the opposition victory will likely give way to some apprehension. Mahathir was also accused of being an authoritarian leader, and political opponents were thrown in jail during his time in office.
But for the opposition, he proved to be the perfect standard-bearer to take on Najib.
He is a staunch supporter of the country's Muslim Malays who could appeal to the country's biggest ethnic group, and whose years in power were remembered as a prosperous period in the country's history.
