Arnulf was the son of Margrave Luitpold of Bavaria and Cunigunda, daughter of Berthold I, the count palatine of Swabia. After the death of his father at the 907 Battle of Brezalauspurc he followed him as ruler of the Bavarian estates around Regensburg.
Besieged by frequent raids by the Hungarians and desperate to raise funds to finance a defence, Arnulf strengthened his power through confiscation of church lands and property, which earned him the nickname "the Bad". He re-established the stem duchy of Bavaria and eventually negotiated a truce with the Hungarians who thereafter largely passed through Bavaria on their raids into other German territories.
Arnulf vigorously resisted King Conrad I, whom he had supported in the course of his election and who in 913 had married his mother Cunigunda. In Conrad's conflict with Erchanger and Burchard II of Swabia he backed his Swabian cousins and later challenged Conrad's successor, Henry the Fowler of Saxony. According to the Annales Iuvavenses, in 920, Baiuarii sponte se reddiderunt Arnolfo duci et regnare ei fecerunt in regno teutonicorum: the Bavarians, with some other East Franks, elected Arnulf king in opposition to Henry (actually in 919). Arnulf's "reign" was short-lived. Henry defeated him in two campaigns in 921, confirmed his sovereignty over Bavaria in return for Arnulf's renunciation of his royal claim.