Yamassoum received a doctorate degree in political science from the University of Bordeaux in France, with a thesis entitled Contribution à l'étude des stratégies et techniques d'influence des États-Unis et de l'Union soviétique en Afrique sub-saharienne (A contribution to the study of American and Soviet strategies and techniques of influence in sub-Saharan Africa).

[6] He served as Prime Minister until June 12, 2002,[7] when he resigned and was replaced by Haroun Kabadi.

Yamassoum was appointed as chairman of the Board of the Hydrocarbons Company of Chad (Societe des Hydrocarbures du Tchad, SHT) in November 2007.

He headed the court when it validated the results of the April 2016 presidential election, in which Déby won re-election in the first round, on 4 May 2016.

[12] In 2019, he became the first chairman of the newly created Central African Financial Market Supervisory Commission, a position he held until 2023.