Léon was born in 1965 in Saint Germain du Teil, Lozère near the Aubrac plateau. He spent his childhood in La Canourgue and attended school in Rodez and Clermont-Ferrand, then in École Sainte Geneviève, Versailles. He received the Diplôme d'Ingénieur de l'École Polytechnique (X84) in 1987, the Magistère de Mathématiques Fondamentales et Appliquées et d'Informatique from École Normale Superieure in 1988, the Diplôme d'Études Approndies in Computer Science in 1988, and a Ph.D. in Computer Science from LRI, Université de Paris-Sud in 1991.
After his Ph.D., Léon joined AT&T Bell Laboratories from 1991 to 1992. He then became chairman of Neuristique, a small company pioneering machine learning for data mining applications. He returned to AT&T Labs from 1995 to 2002, and NEC Labs America at Princeton from 2002 to March 2010. He joined the Science team of Microsoft adCenter in 2010 and Microsoft Research in 2012.
Léon's primary research interest is machine learning. His contributions to this field address theory, algorithms and large scale applications. Léon's secondary research interest is data compression and coding. His best known contributions are his work on large scale learning and on the DjVu document compression technology. Léon has published over eighty papers. He is serving or has served on the boards of the Journal of Machine Learning Research, IEEE Transactions on Pattern Analysis and Machine and Pattern Recognition Letters.