In 1881 he wrote a thesis De la ponctuation and was employed in the École Nationale des Chartes as an archivist. He became general inspector in the Bibliothèque nationale de France. He participated in the compilation of the "general catalogue of the manuscripts of the public libraries of France" (Alençon, Avranches, Louviers). At the same time, he undertook research on ancient libraries and the history of printing and books. Omont was a member of the École Nationale des Chartes and Societé des Antiquaires de France. He was elected as a member of the Académie des Inscriptions et Belles-Lettres in 1900. After his death his private library stayed with his widow till it was bought in 1948 by the Catholic University of Leuven, to reconstruct its collections after they were destroyed by the Germans for a second time. Omont's library was divided with the partition of the university in 1970 in a Dutch-speaking university, that remained in Leuven, and a French-speaking one, that moved to a new university town called Louvain-la-Neuve.
Bibliographie des travaux de M. Henri Omont. Paris: H. Didier ; Toulouse: Ed. Privat, 1933, XI-270 p. 1108 entries. "Pour le cinquantième anniversaire de l'entrée à la Bibliothèque nationale de m. Henri Omont, la bibliographie de ses travaux a été dressée par les conservateurs-adjoints et les bibliothécaires du Département des manuscrits de la Bibliothèque nationale." Edited by P. Lauer and E. A. van Moé.
Chris Coppens a.o.(eds), Sapientia aedicavit sibi domum: Leuven University Library 1425-2000, Leuven 2005, p. 351