Saleh was born in Biltine, Chad. He is from Wadi Fira Region in eastern Chad and is a member of the Maba ethnic group. He studied Sciences (Mathematics) at the University of Orleans in France and obtained his Phd in 1978. He was a leader of the student movement named FEANF (Federation of Black African Students in France) and a leader of the first movement of rebellion in Chad named FROLINAT. He was also a professor and researcher in mathematics in France, Algeria, Niger, Chad (at the University of N'Djamena).
During Hissène Habré's presidency (1982–90) Ibni Oumar was Minister of Livestock, Higher Education, Scientific Research and Cooperation and Planning (1982-1990), and under his successor Idriss Déby he was Rector of the University of Chad and continued to hold ministerial positions (Cooperation 1991-94), Advisor in developpements answers. Saleh founded a political party, the Party for Liberties and Development (Parti pour les Libertés et le Développement, PLD), in 1993.
At the present time Ibni Oumar Mahamat Saleh is Secretary-General of PLD and spokesperson of the CPDC (Coordination of Political Parties for the Defense of the Constitution), which groups 21 parties. He was the PLD candidate in the May 2001 presidential election; the fairness of the election, which was won by incumbent president Idriss Déby, was hotly contested by Saleh and the other opposition candidates. Saleh was arrested at this time. He boycotted the parliamentary election of 2002, the constitutional referendum of 2005 and the presidential election of 2006, as he claimed there were not sufficient guarantees of a free and fair consultation.