He replaced former president Robert Mugabe, whose resignation Tuesday after military and public pressure ended his 37-year stay in power, Xinhua news agency reported.
Mnangagwa became Zimbabwe's second president since the nation attained independence from Britain in 1980.
The 75-year-old former vice president returned to Harare Wednesday after being sacked by Mugabe on Nov. 6.
His sacking led the military to move in and kick off a series of extraordinary events ending with Mugabe stepping down Tuesday amid impeachment proceedings.
Taking his oath of office before Chief Justice Luke Malaba, Mnangagwa pledged to uphold and defend the constitution and the rights of Zimbabweans.
"I will promote whatever that will advance and oppose whatever that will harm Zimbabwe. I swear that I will protect and promote the rights of the people of Zimbabwe," he said.
The Commander of the Zimbabwe Defense Forces (ZDF) Constantino Chiwenga on Friday pledged allegiance to the new president in line with the country's constitution.
The swear-in ceremony at the National Sports Stadium in Harare was characterized by scenes of jubilation, gun salute, fly past and a constitutional military parade.
The inauguration was graced by dignitaries including Zambian President Edgar Lungu, Botswana President Ian Khama, Mozambican President Filipe Nyusi and Namibia Vice President Nickey Iyambo.
Zimbabwe's opposition leaders also attended the event.