After being named to the Iraq post in June, the Brazilian diplomat said his top priority was to protect the interests of the Iraqi people under the U.S.-led occupation.
"I have been sent here with a mandate to assist the Iraqi people and those responsible for the administration of this land to achieve ... freedom, the possibility of managing their own destiny and determining their own future," he said on arrival in Baghdad in June.
All the national flags that ring the U.N. headquarters' entrance in New York were removed from their poles. The blue and white U.N. flag was lowered to half staff.
U.N. Secretary-General Kofi Annan said Vieira de Mello was "an outstanding servant of humanity."
"The loss of Sergio Vieira de Mello is a bitter blow for the United Nations and for me personally," Annan said in a statement. "I can think of no one we could less afford to spare."
Vieira de Mello's spokesman in Baghdad, Salim Lone, said, "He was a wonderful guy. He was the U.N. in a way."
"Wherever there was suffering he was there," he said. "He was everywhere. He was a very brilliant man, totally wedded to the United Nations."
The Iraq posting capped a career as the United Nations' crisis pointman, sent to conflict zones from Kosovo to Cyprus to East Timor to help end the bloodshed and rebuild in the aftermath. Since September, he served as the U.N. High Commissioner for Human Rights, taking leave from the post to serve in Baghdad.
There, he had to rely on both diplomacy and tough talk as the United Nations tried to find its place after the Iraqi war come close to rendering it obsolete. He had to work with the United States, even while distinguishing the world body from the occupation, unpopular with many Iraqis.
In his last interview published before his death, Vieira de Mello sympathized with Iraqi resentment at having foreign troops on their soil.
"It is traumatic. It must be one of the most humiliating periods in their history. Who would like to see their country occupied? I would not like to see foreign tanks in Copacabana," he told the Brazilian newspaper Estado de Sao Paulo in an interview published Monday.
Throughout his tenure in Baghdad, he took pains to remind everybody that the United Nations would be in the country long after U.S. forces leave and insisted that the world body — not the U.S.-led coalition — should control the spending of Iraqi oil revenues.
"We're truly in a unique situation here," he said of the occupation. "By the usual U.N. standards, this is at best a bizarre situation."
Born in Rio de Janeiro on March 15, 1948, Vieira de Mello studied philosophy at the prestigious Sorbonne University in Paris before embarking on a career in the United Nations, rapidly gaining a reputation as a smooth-talking, hands-on operator.
He joined the U.N. High Commissioner for Refugees in 1969, serving in Bangladesh during its independence in 1971. Following the 1974 Turkish invasion of Cyprus he worked with refugees on the island.
He spent three years in charge of UNHCR operations in Mozambique during the civil war that followed its independence from Portugal in 1975, and three more in military-ruled Peru.
He became senior political adviser to the U.N. Interim Force in Lebanon between 1981 and 1983, covering the period when Israel invaded.
Vieira de Mello returned to the UNHCR, working at its head office in Geneva for 10 years.
The early 1990s found him in Cambodia and then in disintegrating Yugoslavia. After working on the refugee problem in central Africa, in 1996 he was made assistant high commissioner for refugees and in 1998 became undersecretary-general in New York.
He was a special U.N. envoy in Kosovo following the U.S.-led bombing raids that broke Serbian control of the province in 1999.
When Indonesia withdrew from East Timor later that year, Vieira de Mello was sent there. He arrived in the tiny island territory in the far reaches of the Indonesian archipelago shortly after a rampage by pro-Jakarta forces left it in ruins.
"I felt like an ambulance driver arriving at the site of a car crash and finding a dismembered body in a state of clinical death," he recalled shortly before East Timor gained its independence in May 2002.
Nevertheless, he gained widespread praise for overseeing the country's three-year transition to independence.
"You don't change the devastation of 1999 into a Garden of Eden in 2½ years," he said, but added, "we have laid solid bases for the country to live in peace."
Vieira de Mello was divorced and had two sons. His mother lives in Rio de Janeiro.