“We are deeply distraught, devastated as a country,” Mahama said after his swearing-in ceremony.
The cause of Mills’ death has yet to be determined. He died three days after his 68th birthday.
Ghana, looked at as one of Africa’s most progressive democracies, rushed to make Mahama president out of fear of a possible coup as has happened in other countries.
Throughout the presidential campaign, Atta Mills’ opponents claimed he was not healthy enough to run for a second term, and even broke a report that he had falsely died during a trip to the United States when he hadn’t.
He was elected in 2008 in a tight race. It was his third presidential bid.