[1] Arnulf was the son of Margrave Luitpold of Bavaria and Cunigunde, herself a member of the Ahalolfing dynasty, daughter of Berthold I, the count palatine of Swabia.

After the death of his father, Arnulf succeeded him in his Bavarian lands, and soon after he assumed the title of a "Duke of Bavaria"[2] as ruler of the estates around Regensburg.

An energetic and combative man, he received broad support from the local nobles, however, already during his ascension in to dukedom, he was faced with constant raids from the Hungarians.

[3] Besieged by frequent Hungarian raids and desperate to raise funds to finance a re-organized defense, Arnulf strengthened his power through confiscation of church lands and the secularization of numerous monastery estates, which earned him the nickname "the Bad" by medieval chroniclers.

In September, the king convoked a church council in Hohenaltheim, attended by the Bavarian episcopate, which summoned Arnulf and his younger brother Berthold on the grounds of excommunication at Regensburg on 1 November.

In January 917, King Conrad, angered at the situation, called for the execution of his rebellious Swabian brother-in-law, Erchanger and his brother Berthold, giving Arnulf more pause for concern.

[5] On his return, according to the Annales iuvavenses, in 920, Baiuarii sponte se reddiderunt Arnolfo duci et regnare ei fecerunt in regno teutonicorum ("the Bavarians freely submitted themselves to Duke Arnulf and asked him to reign within the realm of the Germans").

In 935 he launched an attack against King Hugh of Provence to obtain the Iron Crown of Lombardy for his eldest son Eberhard, which only resulted in defeat.