His name appears in a list of monks of Saint-Germain-des-Prés written around 841/847 (a declaration of spiritual association with the monks of the Abbey of Saint-Remi). In 858, he went to Spain with his colleague Odilard monk to collect relics; they returned with those martyrs George, Aurelius and Nathalie, Christians executed in Córdoba, Andalusia on 27 July 852. The account of this voyage, accompanied by miracles, was told by their colleague Aimoin
Usuard is the author of a martyrology dedicated to Charles the Bald. The dedication seems to only briefly precede the death of the author. The martyrology of Usuard enjoyed consistent success throughout the Middle Ages, as evidenced by numerous surviving manuscripts. This martyrology synthesizes elements of the old Martyrologium Hieronymianum, the martyrology of Ado of Vienne, and an enhanced version of that of Bede, attributed to the archdeacon Florus of Lyon. It contains eleven hundred saints' lives.
Usuard also composed an obituary of the abbey of Saint-Germain-des-Prés, the oldest of its kind (edited by Auguste Molinier, The French obituaries in the Middle Ages, 1890).
- Henri Quentin gives a partial list: cf. Martyrologes historiques, 1908, pp. 675-677.
- Henri Quentin, Les martyrologes historiques du Moyen Âge, Paris, 1908.
- Dubois, Jean (1957). "Un témoin de la vie intellectuelle à Saint-Germain-des-Prés au IXe siècle: Le martyrologe d'Usuard". Revue d'histoire de l'Église de France. 43 (140): 35–48. doi:10.3406/rhef.1957.3214. ISSN 0300-9505. Retrieved 2016-11-02.
- Overgaauw, E.A. (1988–1989). "Les deux recensions de la lettre-préface d'Usuard à Charles le Chauve et les trois recensions de son martyrologe". ALMA, Bulletin du Cange. 48–49: 85–101. hdl:2042/3414.