UN Secretary General Kofi Annan had appointed the Brazilian diplomat last month as the United Nations’ special representative for Iraq.
Annan gave him the job of improving the lives of ordinary Iraqis suffering from the anarchy which followed the fall of Saddam Hussein's regime.
But critics say de Mello's role was undermined by the Americans, and his very presence in Iraq legitimised the US occupation.
Born in Rio de Janeiro in 1948, de Mello joined the UN while studying philosophy and humanities in his home city and at Paris' Sorbonne University.
He spent the majority of his career working for the UN's High Commissioner for Refugees office in Geneva, and served in Bangladesh, Sudan, Cyprus, Mozambique, Peru and Yugoslavia.
He was later appointed Deputy High Commissioner for Refugees, before being promoted to the position of Under Secretary for Humanitarian Affairs.
De Mello’s finest moment occurred when he took the lead in the UN's operations in East Timor.
Critics say de Mello was
undermined by the Americans
Commentators say his successful overseeing of the territory's fractious transition from Indonesian province to independence was his greatest achievement.
But his job in Iraq proved more difficult.
De Mello said the UN would "assist Iraqis any way that we can", but the UN's power in Iraq is still a subject of some debate.
While Paul Bremer, Iraq's occupation administrator, had pledged to work with de Mello, the extent of the Brazilian's involvement was unclear.
For example, there was no obvious participation on his part in the formation of the governing council for Iraq.
De Mello's responsibilities included working with the United States and Britain to coordinate humanitarian and reconstruction assistance, and to "facilitate a process leading to an internationally recognised representative government of Iraq."
During his four-month term, he was also expected to have had a seat on an international advisory board created to monitor Iraqi oil sales to fund reconstruction efforts.
But asserting the UN's desired role in Iraq has proved difficult against occupying forces keen to retain the final say.