He said he felt “deeply humbled” to take the role.
And he said he was “not oblivious to the many Zimbabweans from across the political and racial divide who have helped make this day.”
He paid tribute to his predecessor Robert Mugabe – to muted applause – calling him “a father, mentor, comrade-in-arms and my leader”.
Mr Mugabe left office dramatically this week after 37 years of authoritarian rule. His departure followed a power struggle in which Mr Mnangagwa was sacked as vice president to pave the way for Grace Mugabe, the then-first lady, to take up the presidency.
Mr Mnangagwa fled the country but returned to a hero’s welcome and on Friday struck a conciliatory tone.
“The task at hand is that of rebuilding our country,” he said.
“I am required to serve our country as the president of all citizens regardless of colour, creed, religion, tribe, totem or political affiliation.”
Although Mr Mnangagwa has unseated Zimbabwe’s long-time ruler, he is still associated by many with some of the worst atrocities committed under the ruling Zanu-PF party since the country gained independence in 1980.
He was the country’s spymaster during the 1980s civil conflict, in which thousands of civilians were killed. But he has denied any role in the massacres, blaming the army.
Tens of thousands of people packed the National Sports Stadium in Zimbabwe’s capital, Harare, to witness the inauguration. Pop singer Jah Prayzer provided the entertainment and, as people in the crowd danced, the atmosphere was closer to that of a concert.
Dignitaries, including leaders from various African countries, filed in to cheers.
Opposition leaders Morgan Tsvangirai and Joice Mujuru – who have both also had their sights on the presidency at various times – were there. (Courtesy BBC)