A 34-year career in the United Nations came to a terrible end as a truck bomb outside Vieira de Mello's Baghdad office brought the building tumbling down on top of him.
"The spirit, the vision, the optimism, and the energy of Sergio
will remain in Iraq," said the Mexican Ambassador to the United Nations, Adolfo Aguilar Zinser.
A former UN High Commissioner for Human Rights, Vieira de Mello was a Brazilian diplomat well-known for his steady pragmatism.
His final fatal mission in Iraq required Vieira de Mello to tactfully assert the UN's role in Iraq against occupying forces keen to retain their power – a job he was only just coming to grips with.
In late 2002, he was made head of the UN's Human Rights Commission, a tough and often controversial job.
Born in Rio de Janeiro in 1948, he joined the UN while studying philosophy and humanities in France’s Sorbonne University.
Much of his career concerned the UN's High Commissioner for Refugees in Geneva, but also serving UNHCR in Bangladesh, Sudan, Cyprus, Mozambique, Peru and Yugoslavia.
First major role
In 1981, he became the principal adviser to UN forces in Lebanon – a responsibility he took on for three years.
But other important and difficult jobs included humanitarian coordination after the Rwandan genocide in the mid-1990s when Vieire de Mello spent several months working in the Great Lakes region in 1996.
He also worked as Secretary General Kofi Annan's special representative for Kosovo.
1981: Adviser to UN forces, Lebanon
1996: Humanitarian coordinator, Rwanda
1999: Special representative for Kosovo
2000: Head of UN operations, East Timor
East Timor mission
Vieira de Mello headed the UN's operations in East Timor, overseeing the territory's problematic transition from Indonesian province to independence – widely seen as his greatest achievement.
East Timor's Foreign Minister and Nobel peace prize winner Jose Ramos Horta Tuesday said he felt deep dismay at the death of the man who helped build his nation after independence.
"I express my deep dismay, on behalf of myself and on behalf of the people of Timor, at the tragic attack which has hit the United Nations and Sergio Vieira de Mello," Ramos Horta told the Lusa press agency in Dili on hearing the news that the envoy had been injured.
Ramos Horta said the Timorese knew Sergio Vieira de Mello well, adding "he won our friendship and respect, and we are deeply dismayed ... by this tragic and repugnant act".
He was later reported to have told BBC radio his country's flag would be hung at half staff to mark the death of the UN secretary general's representative in Iraq.
Following a 1999 referendum Sergio Vieira de Mello was named by the UN as the temporary administrator of East Timor, tasked with rebuilding the territory which had been devastated by pro-Indonesian militias, until the nation gained independence from Jakarta in May 2001.
Like Iraq, the mission was not without its dangers - a visiting correspondent from the South China Morning Post recalled a poster on the wall of Vieira de Mello's office requesting visitors to unload their weapons.
While the US representative to Iraq, Paul Bremer, pledged in June 2003 to work with Vieira de Mello to improve the lives of ordinary Iraqis - the extent of the Brazilian's involvement is unclear.
There was no obvious participation on his part in the formation of the new governing council for Iraq.
The UN's power in Iraq is still a subject of debate, Vieira de Mello said that the UN would "assist Iraqis any way that we can".
Source: Aljazeera + Agencies