The thumping victory capped a dramatic political comeback for Mahathir, who previously ruled the country with an iron fist for 22 years and came out of retirement to take on Prime Minister Najib Razak after the leader became embroiled in a massive corruption scandal.
As no single party registered a majority, it is up to the Malaysian king to decide who will form the cabinet.
A flag-waving crowd of supporters gathered on a field outside the headquarters of Mahathir's party near Kuala Lumpur. Almost 14.5 million Malaysians were eligible to vote at the country's 8,253 polling stations, as per Malaysia media reports.
The Star online, owned by the Star Media Group, said "Dr M" expects to be sworn in at 5pm, while the New Straits Times reported how outgoing PM Najib Razak was ready to accept the choice of Government made by the the king.
"Now the hard work begins", influential political website Malaysiakini.com said in an editorial. "We need to attend to this mess as soon as possible".
Najib, who was seeking a third term after coming to power in 2009, has been dogged by allegations of corruption as at least 10 countries investigate the US$4.5 billion missing from a state investment fund known as 1Malaysia Development Berhad (1MDB). While Najib denied wrongdoing, Mahathir lambasted him as a "thief" on the campaign trail. "I am not seeing revenge, we don't want to punish people but the rule rule of law will be clearly implemented", he said.
Bridget Welsh, an analyst from John Cabot University, slammed his speech as "unstatesmanlike". "Mahathir was a game changer".
In Mahathir, the opposition found the ideal person to take on Najib. "They wanted Najib gone".
Australian Prime Minister Malcolm Turnbull said he was looking forward to working with the Malaysian government following the election results.
Malaysia's currency weakened in offshore trading on Thursday, with the ringgit one-month non-deliverable forward falling 1.7 pct.
"I can not accept this country being destroyed by selfish people who only think about themselves. who steal money", he said, referencing the 1MDB corruption scandal which has dogged Najib's premiership. Sustaining that kind of momentum will not be an easy task, particularly as the opposition transitions from campaigning to governing - with Mahathir claiming he will eventually step aside - and supporters of the defeated ruling coalition begin realigning post-Najib using their deep patronage networks and other levers of influence. The GST helped inflation accelerate previous year at almost the fastest pace in a decade.
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"Malaysian opposition pledge to reintroduce fuel subsidies would distort market-determined price mechanisms, impacting fiscal position and balance of payments".
Pakatan Harapan's economic policies, if left unadjusted, may be credit negative for Malaysia's sovereign debt rating.
That saw the opposition make strong gains in long-time BN strongholds in the polls, including Sarawak state on Borneo island, and the state of Johor, the birthplace of UMNO.
For Mahathir, the fight against his former protege was particularly ugly.
Barisan Nasional did not cheat in the general election, said its Chairman Najib Tun Razak. "But for as long as I can contribute, I will continue, and I will back Anwar if that is the wish of the party".
"No such meeting took place", said the 64-year-old leader, calling the election a "tough clash" marked by "slander" against his government.
"We expect today for me to be sworn in as prime minister", he said, addressing concerns over a delay. He also faces a five-year ban from politics.
"There were defamatory and seditious statements that said Barisan cheated during the general elections", said Najib in an address at Putra World Trade Centre (PWTC) this morning. "It's going to be a headache".
"We are once again Malaysians regardless of races".
"I feel that with this change we probably can see something better in the future ... our hope for the future is a better government, fair, free and united", he said.