But yesterday, in what many expected to be a publicly televised resignation, the world's oldest head of state instead delivered a meandering speech, making it clear that he had no intention of leaving the presidency.
Analysts said military commanders may have worked out a deal that would lead to Mugabe's resignation after an interim period and his replacement by Mnangagwa, possibly during next month's congress of the ruling Zanu-PF party. For now, the military's vague statement about a "road map" left plenty of room for conjecture.
What is clear is that as long as Mugabe remains in power at the military's behest, millions here will be devastated. For years, as Mugabe's rule grew more erratic and repressive - and as the economy continued to collapse - Zimbabweans spoke openly about when and how the "old man" would go. This week brought that outcome closer than ever.
Mnangagwa's return would appease a small but powerful segment of the ruling party. He has been a core member of Zanu-PF for decades, with strong connections to the security forces. But many Zimbabweans see him as corrupt and oppressive for having helped insulate Mugabe's regime for years before their abrupt falling-out.
Chiwenga's statement said Mnangagwa was "expected in the country shortly". He fled to South Africa after being fired, most likely to avoid the possibility of arrest.
Mugabe could still be forced out through constitutional channels. Tomorrow, lawmakers were expected to begin proceedings to impeach him, although it remained unclear how the military's statement would affect those plans. Some MPs who support Mnangagwa could now back away from impeachment.
Impeachment could take days or months, depending on which constitutional lawyer is consulted. It could be a unanimous vote, or a deeply divided one, depending on how opposition parties decide to act.
While some have looked to other African coups - such as those in Burkina Faso, the Central African Republic and Nigeria - for insights into how things might play out in Zimbabwe, this has become a wholly different transition of power.
Both Mugabe's military and civilian opponents are tiptoeing around the country's laws, eager to imbue any successor with an air of legitimacy that would be accepted by the international community. In its bylaws, the regional bloc of southern African nations includes strong language against coups.
If the military has indeed agreed to allow Mugabe to remain in power in exchange for its own demands, it has done so as much by diplomacy as force. In his speech yesterday, Mugabe told the nation that the military operation that led to his detention "did not amount to a threat to our well-cherished constitutional order".
For years, Mugabe took the law into his own hands. But when Mugabe's own government finally turned against him, it declined to use the same brute force or extrajudicial power he has employed for years.
Aside from wanting to avoid allegations of coup-plotting, Zimbabwe's military may have been showing deference to the only president Zimbabwe has ever had and a hero of the country's liberation struggle. In recent years, many critics of Mugabe's leadership have blamed his wife, Grace Mugabe, who is often portrayed as a puppet master manipulating a senile man. Grace Mugabe was seen as angling to be her husband's successor in the wake of Mnangagwa's dismissal.
Over the next few days, Mugabe's opponents have said, they plan to explore legal channels, such as impeachment, to remove him.
"For years we've been victims of the lawlessness of the ruling party," said Lovemore Madhuku, a professor of constitutional law at the University of Zimbabwe. "If we abandon the law to get Mugabe out, we are not safeguarding ourselves from more lawlessness in the future."
Amid the uncertainty, a debate is raging among legal experts and lawmakers about how long an impeachment process would take. Paul Mangwana, deputy secretary of Zanu-PF said it would take only two days. Madhuku said it would likely take months if the law were followed.
Mangwana said Parliament would set up a committee responsible for impeachment tomorrow and that it would issue its decision on Thursday.
"The main charge is allowing his wife to usurp government powers," he said.
But after the military's announcement, it was unclear whether impeachment proceedings would move forward. If Mnangagwa is permitted to return to the country as a part of a military-led compromise, some anti-Mugabe members of Zanu-PF might back down from their impeachment demands, accepting a compromise that would keep Mugabe in power.
Yesterday, the party voted to remove Mugabe as its leader and expelled his wife for life, so accepting his role as head of state now would be a stunning about-face.
