Arnulf was the illegitimate son of the Bavarian king Carloman and his concubine, Liutswind (Litwinde, Litwindie) (of Bavaria), of Carinthian origin, daughter of Count Ernst.
Upon the deposition of his uncle, the emperor Charles III the Fat, Arnulf assumed his title of King of the East Franks in 887. Soon after his election for a king the term Karantania became a new European political term.
In Carinthia Carloman had a court in Moosburg (Blatograd), where young Arnulf spent his childhood. From later events it is evident that the Karantanians treated him as their own duke.
Arnulf didn't negotiate but he fought. At the decisive Battle of Leuven on September 891 he defeated an invading force of the Northmen, or Vikings, essentially ending invasion attempts on that front and the kingdom. After his victory, Arnulf had built a new castle on an island in the Dijle river (Latin Luvanium, local Lovon). The Annales Fuldensis (Annals of Fulda) report that the bodies of dead Northmen blocked the run of the river.
In 894/895 Great Moravia probably lost a part of its territory - present-day Western Hungary- to him. Arnulf failed to conquer Great Moravia in 892, 893, 894/895 and 899.
In 895 Bohemia broke away from Great Moravia and became his vassal. An accord was made between him and the Bohemian Duke Borivoj (reigned 870-895), Bohemia was freed from the danger of invasion.
Arnulf invaded Italy in 896 and was crowned Emperor by the Pope Formosus (pope 891-896). On his death in 899, he was succeeded as a king of the East Franks by his son, Louis the Child (900-911).
Arnulf has a bastard son Zwentibold, who reigned as King of Lotharingia from 895 to 900.