Figures for Sunday's second-round ballot released on live television by the interior ministry showed Keita won an overwhelming 77.6 per cent of the vote, with his rival Soumaila Cisse trailing on 22.4 per cent.
Mali's first election since 2007 was seen as crucial for unlocking more than USD 4 billion (three billion euros) pledged by international donors. Aid to the country had been halted after a separatist uprising led to a 2012 coup and an Islamist insurgency that plunged the country, then seen as one of the region's most stable democracies, into turmoil.
Turnout was recorded at 45.8 per cent, Interior Minister Moussa Sinko Coulibaly said yesterday, while 93,000 ballots were spoiled, compared with 400,000 in the first round.
Keita, 68, has a reputation for toughness and is known for his blunt speaking and unwillingness to compromise.
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His regime begins in controversy after it emerged on Wednesday that Captain Amadou Sanogo, who led a group of fellow mid-level officers to overthrow then-president Amadou Toumani Toure on March 22 last year, had been promoted.
And Human Rights Watch described Sanogo's elevation from captain to lieutenant-general as "outrageous".
"Sanogo and forces loyal to him have been implicated in extremely serious abuses, including arbitrary detention, enforced disappearances, attacks against journalists and torture," it said.
"Instead of being rewarded with this promotion, Sanogo should have been investigated for his alleged involvement in these acts."
During his campaign, Keita vowed to unify Mali after the humiliation of having to call on former colonial power France in January to help repel the Islamist insurgency in the north, where Al-Qaeda-linked movements seized key towns in the chaos following Sanogo's coup.