Figures for Sunday’s second-round ballot released on live television by the interior ministry showed the ex-prime minister won an overwhelming 77.6 per cent of the vote, with his rival Soumaila Cisse trailing on 22.4 per cent.
The country’s first election since 2007 was seen as crucial for unlocking more than $US4 billion 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.
Mr Cisse, a former finance minister, had already conceded defeat after it became apparent as early as Monday that victory was beyond his grasp.
Turnout was recorded at 45.8 per cent, the interior minister said, while 93,000 ballots were spoiled, compared with 400,000 in the first round.
Mr Keita, 68, has a reputation for toughness and is known for his blunt speaking and unwillingness to compromise.
Coup leader Sanogo promoted
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.
“My euphoria re ‘quality’ of Mali’s democracy is waning,” Robert Piper, the United Nations’ regional humanitarian coordinator for the Sahel region, posted on Twitter.
Human Rights Watch described the coup leader’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.”
Keita vows to unify fragile Mali
During his campaign, Mr 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.
Al Qaeda-linked movements seized key towns in the chaos following Captain Sanogo’s coup, but were driven out following nine months of occupation.
“For Mali’s honour, I will bring peace and security,” Mr Keita has said.
“I will revive dialogue between all the sons of our nation and I will gather our people around the values that have built our history: dignity, integrity, courage and hard work.”
Mr Cisse, 63, had complained of widespread electoral fraud before conceding defeat but told reporters this week he would not challenge the result in court “in view of the fragility of the country”.