He was United Nations Special Rapporteur on contemporary forms of racism, racial discrimination, xenophobia and related intolerance from 2002–2008.

In 1977, he joined the UNESCO secretariat, where he held several positions including Director of the Division of Inter-cultural Projects.

He was appointed Special Rapporteur for racism-related topics by the United Nations Commission on Human Rights in August 2002, replacing Maurice Glele-Ahanhanzo of Benin and serving until July 2008 when he was succeeded by Githu Muigai (Kenya).

[2] He also investigated, as an independent expert, human rights violations during the 2010–2011 Ivorian crisis, and later in Gaza strip in 2014 to 2018.

[3] In 2007, Diène denounced the words of French President Nicolas Sarkozy during his "Dakar speech" and spoke of an attempt to, "scientifically legitimize historical stereotypes of the construction of racism".