He was educated at the imperial court and became lay-abbot of St Riquier in commendam. He served his cousin Charles the Bald in both war and peace, carrying out two missions to Lothar during the Carolingian Civil War and fighting at Fontenoy in June 841. It is probable that he died as the result of wounds received whilst fighting for him against the Northmen near Angoulême. The date of his death is disputed among scholars, but consensus is now for June 14, 844 . In the 11th century his body, with the fatal wound still visible, was found in the grave of his father, Angilbert.
Nithard's historical work consists of four books on the history of the Carolingian empire under the turbulent sons of the emperor Louis I, especially during the turbulent period between 838 and 843. The Historiæ or De dissensionibus filiorum Ludovici pii (On the Dissensions of the Sons of Louis the Pious) is valuable for the light which it throws upon the causes which led to the disintegration of the Carolingian empire. The first three of these books were written before Nithard's appointment as lay-abbey of St-Riquier in winter of 842, the fourth and final in spring of 843 after taking up office there. Although rough in style, partisan in character and sometimes incorrect in detail, the books are the work of a man who had an intimate knowledge of the events which he relates, who possessed a clear and virile mind, and who above all was not a recluse but a man of action. They are dedicated to Charles the Bald, at whose request they were written.
Only two manuscripts of the Historiæ survived, one roughly contemporary and an incomplete Renaissance-era text useless in the reconstruction of the text.