Mnangagwa said that Zimbabwe has shown it is a "democracy with maturity" during the ceremony at the National Sports Stadium in the capital Harare.
The 80-year-old said he would ensure that the country's constitution and rule of law were upheld over his second term.
He was declared the winner after a presidential election that observers deemed insufficiently free and fair.
Several human rights organisations pointed to voter intimidation, the falsification of the electoral rolls and the appointment of backers of the governing party Zanu-PF to the electoral commission.
The country's main opposition party also rejected the results, saying the elections had been marred by "voter suppression and egregious abuse".
According to the electoral commission, Mnangagwa received almost 53 per cent of the votes cast while the most promising opposition candidate, Nelson Chamisa of the Citizens Coalition for Change (CCC) party received 44 per cent.
Mnangagwa's governing Zanu-PF party also recorded a solid majority in parliament.
Zimbabwe's elections have been marred in the past by incidents of violence and allegations of manipulation.
The country has been mired in an economic crisis for decades, with soaring inflation and unemployment.
Zimbabwe was ruled by strongman Robert Mugabe from 1987 to 2017.
After Mugabe was overthrown by the military, former vice president Mnangagwa came to power.
A year later, the elections won by Mnangagwa were also overshadowed by violence and repression.
A former British colony, Zimbabwe has been governed by the Zanu-PF party since it became independent in 1980.
