Arnulf gave distinguished service at the Austrasian court under Theudebert II (595-612). In 613, however, with Pippin of Landen he led the aristocratic opposition to Queen Brunhilda of Austrasia that led to her downfall and the reunification of Frankish lands under Clotaire II. About the same year, he became bishop of Metz.

From 623, again with Pippin, now mayor of the Austrasian palace, Arnulf was adviser to Dagobert I, before retiring in 627 to become a hermit.

Arnulf's parentage is uncertain: some have claimed that his father was Arnoldus (b Abt. 535, Saxony, Germany - d. 600, and that his mother was Ada of Schwabia. According to Frankish myth, Arnulf was the son of Bodigisel.