Illegitimate son of Karloman, King of the East Franks, and his mistress Liutswindis.
The Annales Ducum Bavariæ record the death in 880 of "Karlomannus rex" leaving "regnum cum ducatu Bawarie" to "Arnolfo, filio suo ex concubina nobili de Karinthia". "Arnolfus…rex" donated the abbey of Herrieden to Eichstätt by charter dated 23 Feb 888 in which he names "Karlomanni patris nostri". He is called "Arnulfum filium spurium Carolomanni" when recording his accession in 887. He was invested with the March of Pannonia and Carinthia in . During the illness of his father, he administered Bavaria but was obliged to transfer the territory to his uncle Ludwig III on his father's death, in return for receiving the duchy of Carinthia.
He led a powerful army of Carinthians and Slavs against Emperor Karl III in early Nov 887, and 27 Nov 887 issued his first charter as ARNULF King of the East Franks. Reuter highlights the absence of evidence concerning the manner in which Arnulf assumed power in place of his uncle. Fulco Archbishop of Reims came to Worms in Jun 888 to invite him to claim the kingdom of the West Franks, challenging Eudes who had recently been elected king. Arnulf won a decisive victory over the Vikings at the River Dyle in 891, after which Viking raids in the east Frankish kingdom all but ceased.
Called by the Pope to fight Guido I Duke of Spoleto, Arnulf crossed the Alps in 894 and subjugated northern Italy, but was forced to withdraw by Italian resistance. Returning to Italy after Guido’s death in 895, he was crowned Emperor ARNULF at Rome 22 Feb 896 by Pope Formosus. He marched against Lambert of Spoleto, but was struck by paralysis and obliged to return to Bavaria.
Regino records the death "899 III Kal Dec" of "Arnulfus imperator" and his burial "in Odingas ubi et pater eius tumulatus est". The necrology of Regensburg St Emmeran records the death "V Kal Dec" and "VI Id Dec" of "Arnolfus imperator fundator huius loci". 1