A master mediator who was keen on flying the flag for peace around the world -- and was jointly awarded a Nobel Peace Prize with the UN in 2001 for his efforts -- Annan brokered a crucial truce in the bloody aftermath of the Kenyan election violence in 2007.
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One Twitter user wrote on hearing of his death Saturday: "Were it not for you, our country would have been brought down to ashes."
In February 2008, Annan led a mediation effort in Kenya after elections in the east African nation descended into chaos. Violence had broken out after Kenya's former President Mwai Kibaki was declared winner of the 2007 presidential polls against opposition leader Raila Odinga.
Around 600 people were killed after marauding youths armed with spears, bows, arrows and machetes destroyed homes around the town of Eldoret, near the border with Uganda.
Annan and his team got both leaders to agree to a power-sharing coalition that ended the bloodshed.
However, his glittering humanitarian legacy is overshadowed by the Rwanda genocide and the UN's response to the tragedy.
It was 1994 and the battle cries of war were sounding in Rwanda. General Romeo Dallaire, who was leading the UN's peacekeeping efforts in Rwanda, was sending frantic messages back to New York about troubling news he was hearing about impending attacks against the country's minority Tutsi tribe.
But his frequent, desperate pleas for help were ignored and he was ordered to pull out the UN peacekeeping troops by the man in charge at the time -- Kofi Annan.
"We could have actually saved hundreds of thousands. Nobody was interested," Dallaire later told CNN's Christiane Amanpour.
As head of the UN department for peacekeeping, Annan faced criticism about the organization's handling of the 1994 Rwandan genocide that would end up killing around 800,000 people.
Even in his death at age 80, some Rwandans have taken to Twitter to say they cannot forget this blight on what many consider to be an exemplary career.
Rwandan journalist James Munyaneza wrote: "Annan will be remembered as the African head of UN peacekeeping operations who brushed aside pleas from Canadian Gen Romeo #Dallaire, head of UN peacekeepers in Rwanda, to reinforce & authorise the peacekeepers to prevent the Genocide and instead ordered peacekeepers' withdrawal."
In his defense, Annan blamed a reluctance among battle-worn world leaders at the time to send troops into Rwanda. "We would have liked to see a larger force in. I had had situations where I called 82 member states together, trying to get troops. I got zilch," he said in a 2008 interview with CNN.
When Annan visited Rwanda in 1998, he was met with a hostile reception and during a speech said: 'We must and we do acknowledge that the world failed Rwanda at that time of evil,'' he said.
'The international community and the United Nations could not muster the political will to confront it. The world must deeply repent this failure.''
Despite his Rwanda failings, Annan was widely recognized as one who advanced the African agenda greatly during his tenure at the United Nations. He led the organization for two successive five-year terms, beginning in 1997.
Many have lauded his significant contributions to the humanitarian and development issues in Africa, not just through the United Nations but through his work at the Mo Ibrahim Foundation and the Africa Progress Panel.
Annan launched a global campaign at an African leaders summit in 2001 calling for fresh funding to tackle the HIV/AIDS epidemic ravaging major countries in sub-Saharan Africa.
A year earlier, Annan had called on world leaders at a UN Security Council meeting to urgently prioritize the AIDS epidemic, which he said was 10 times deadlier than armed conflict on the continent.
"HIV/AIDS is not only an African problem. It is global and must be recognized as such. But within that international obligation, the fight against AIDS in Africa is an immediate priority, which must be part and parcel of our work for peace and security in that continent," Annan said at the time.
"Kofi Annan was a guiding force for good,'' current UN Secretary-General Antonio Guterres said in a statement.
"It is with profound sadness that I learned of his passing. In many ways, Kofi Annan was the United Nations," Guterres wrote. "He rose through the ranks to lead the organization into the new millennium with matchless dignity and determination.''