Mr Mnangagwa, 75, appears set to scoop victory in the polls after his youthful challenger Nelson Chamisa, 40, and his Movement for Democratic Change (MDC) party faced wide-spread criticisms from voters.
But front-runner Mr Mnangagwa who took over from Mr Mugabe when he was expelled from his Zanu-PF party in 2014, has failed to impress his predecessor who said he will not be voting for the party he headed throughout his 37-year stint in power.
Instead, he hinted that it would be Mr Chamisa who would get his vote.
Mr Mugabe said: “I can’t vote for ZANU-PF, what is left?
“I think it is just Chamisa.”
He added: “For the first time ever we have now a long list of aspirants to power.”
Referring to the huge number of candidates on offer, he said: “I cannot vote for those who tormented me. I will make my choice among the other 22 but it is a long list.”
Mr Mnangagwa was ushered into power after military generals seized political control of the nation when Mr Mugabe was booted out of the government over fraud and corruption allegations.
Mr Mugabe had tried to make his wife, Grace, 57, his successor.
Mr Mugabe is said to be in ill health.
Voters had mocked Mr Chamisa for being “too young” to pose any sort of threat to current Zimbabwe Prime Minister Mr Mnangagwa.
The opposition leader turned his hand to politics after working as a lawyer and pastor and took over the MDC when former leader Morgan Tsvangirai died.
Mr Tsvangirai had made it his life’s work to topple Mr Mugabe.
Voter Amanda Nash told the Financial Times: “He’s too young for it.
“We need someone who has been in government for some time to sort out our problems.”
Some have also blasted Mr Chamisa’s speedy rise to the top of the party and called the politician “ruthless” for stepping into Mr Tsvangirai’s role far too quickly after his death.
Mr Chamisa has also accused ruling party Zanu-PF of rigging the forth-coming election using dirty tactics.
He alleged that officials doctored electoral rolls and printed forged ballots as well as “weaponised” food aid by refusing to help poverty-stricken opposition voters.
He said: “It is in the nature of Zanu-PF to cheat.”