A statement shared by his Twitter account on Saturday described Annan as a "global statesman and deeply committed internationalist".
"During his distinguished career and leadership of the United Nations he was an ardent champion of peace, sustainable development, human rights and the rule of law," the statement read.
"Kofi Annan was a son of Ghana and felt a special responsibility towards Africa," it continued.
According to Al Jazeera, current UN Secretary-General Antonio Guterres said Annan was "a guiding force for good".
"It is with profound sadness that I learned of his passing.
"In many ways, Kofi Annan was the United Nations. He rose through the ranks to lead the organisation into the new millennium with matchless dignity and determination."
But Annan's association with the UN was not without controversy.
Just before becoming secretary-general, Annan served as UN peacekeeping chief and as special envoy to the former Yugoslavia, where he oversaw a transition in Bosnia from UN protective forces to NATO-led troops.
The UN peacekeeping operation faced two of its greatest failures during his tenure: the Rwanda genocide in 1994, and the massacre in the Bosnian town of Srebrenica in July 1995.
Retired Canadian general, Romeo Dallaire, who led UN forces in Rwanda, is alleged to have sent a message to Annan's office, warning of the impending massacre, but Annan reportedly told UN military forces not to take any action.
In both cases, Annan offered apologies, but ignored calls to resign.