Mr Mahathir was named the opposition's prime ministerial candidate on Sunday after weeks of infighting, in an extraordinary turnaround as his heirs in government face a massive financial scandal.
He will be the world's oldest leader if the coalition backing him wins a general election due by August, although analysts believe this is unlikely.
The authoritarian leader known for his acid tongue ruled the country for 22 years until 2003, making him Malaysia's longest-serving premier. He jailed opposition members without trial on security grounds in 1987 and was seen as an authoritarian figure who trampled over human rights.
Even in retirement, he could not resist sniping at his successors. When allegations emerged that huge sums were looted from a sovereign wealth fund set up by current Prime Minister Najib Razak, he broke from the ruling party and established a new political outfit to try and oust a man who was once his protege.
The US Justice Department alleges $US4.5 billion was stolen from the investment vehicle, 1MDB, in a campaign of fraud and money-laundering. Mr Najib denies any wrongdoing and has cracked down hard, purging critics from his government and curbing domestic investigations.
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Heir apparent
The most remarkable aspect of Mr Mahathir's return to frontline politics has been a rapprochement with his former nemesis and jailed opposition leader Anwar Ibrahim, a dramatic shift after the pair's falling out dominated the political landscape for two decades.
Mr Anwar was heir apparent to Mr Mahathir until the premier sacked him in 1998 over political differences, and was then jailed for six years on sodomy and corruption charges.
After being released Mr Anwar led the opposition to its best-ever showing in 2013 elections, but was imprisoned again in 2015 under Mr Najib's government. Mr Anwar has condemned his convictions as politically motivated.
After Mr Mahathir cut ties with ruling party the United Malays National Organisation (UMNO) he sought to mend his broken relationship with Mr Anwar, with the pair meeting in 2016 for the first time in 18 years.
His party, Bersatu, joined the main opposition coalition Pact of Hope, which includes Mr Anwar's party and many other former sworn enemies, and on Sunday he was endorsed as the grouping's candidate at their convention.
'It is ridiculous'
If the opposition ousts the UMNO-led coalition which has ruled Malaysia since independence in 1957, Mr Mahathir's position as premier is supposed to be temporary, with Mr Anwar taking over once he is released from jail and has secured a royal pardon to overturn a ban on politics.
The opposition hopes that Mr Mahathir will boost its chances with Muslim Malays, who make up over 60 per cent of the population, with the rest comprising mainly ethnic Chinese and Indians.
Tian Chua, a lawmaker from Anwar's People's Justice Party, said Mr Mahathir could deliver the knock-out blow to a government already reeling from the 1MDB scandal.
"Mahathir will invoke the glorious days of his 22-year reign and that will draw support from rural Malays, who are disgruntled with the rising cost of living," he told AFP.
But ruling party MPs quickly dismissed his candidacy and social media was flooded with angry comments criticising the decision to put forward a man accused of ruling with an iron fist.
"It is ridiculous. He is over 90 and he started all the problems we are facing," said Facebook user Radha Dulip Singh.
Disorganised and disunited
Analysts said a victory for Pact of Hope looked unlikely. A survey by pollster the Merdeka Centre last month showed the ruling Barisan Nasional (BN) coalition potentially regaining its crucial two-thirds parliamentary majority, needed to amend the constitution.
The opposition has often appeared disorganised and disunited – a far cry from the strong alliance that Anwar led at the 2013 polls – while the electoral system is strongly stacked in BN's favour, with the government long accused of gerrymandering.
Oh Ei Sun, from Malaysian think-tank the Pacific Research Center, said the opposition picked Mahathir as he was "the only viable choice" with Mr Anwar in jail and other potential candidates lacking charisma.
"I don't think it will make much of a difference," he told AFP, adding the opposition would struggle to compete with the ruling coalition's electoral machine.
