Prince Sadruddin Aga Khan has died at age 70, a spokeswoman at his Geneva office said on Tuesday.
Sadruddin was (paternal) uncle to the current Aga Khan, Prince Karim.
Swiss news agency ATS reported Sadruddin died in a Boston hospital in the United States on Monday after a long illness.
Sadruddin, an expert in Islamic art, was head of two Geneva-based environmental groups, the Bellerive Foundation and Alp Action.
Harvard-educated, he served as the United Nations High Commissioner for Refugees from 1965-77 and was coordinator for UN humanitarian and economic assistance in Afghanistan from 1988-90.
In 1966, he became, at 33, the youngest person to hold the post of UN refugee chief and left an 'indelible print' during his 12-year tenure, the UNHCR said in a statement.
"He was at the helm of the UN refugee agency during one of its most difficult periods. This included the 1971 Bangladesh crisis, which uprooted 10 million people, the 1972 exodus of hundreds of thousands of Hutus from Burundi to Tanzania..." it said.
The UN also paid tribute to Sadruddin's 'key role' in finding homes for tens of thousands of Asians expelled from Uganda by former president Idi Amin.
Last year, Newsweek magazine said Sadruddin made an unlikely 'fire-breathing ecowarrior' with his 'Iranian passport, American education, Geneva address and long career as a UN diplomat'.
Among the honours and awards he received during his career were the French Legion of Honour and the UN Human Rights Award and the Dag Hammarskjold Honorary Medal.
"He was a man of passion and conviction," commented French language Swiss Television.