The Vita Sancti Arnulfi names "Arnulfus episcopus prosapia genitus Francorum" but gives no further details of his ancestry. The Gesta Episcoporum Mettensis names "Arnulfus…ex nobilissimo fortissimoque Francorum stemmate ortus", with no further information on his parentage, as ninth bishop of Metz and "palatii moderator". A 9th century genealogy names "beatum Arnulfum episcopum" as the son of Arnold. Settipani points out that this genealogy forms part of a series compiled at Metz, from the late 8th century onwards, which glorify the ancestry of the Carolingian dynasty by establishing descent from the early Merovingians as well as from a family of Roman senatorial origin. Further confusion is added by the Gesta Episcoporum Mettensis which names "Agiulfus" as sixth bishop of Metz, stating that "patre ex nobili senatorum familia orto, ex Chlodovei regis Francorum filia procreatus", and that "nepos ipsius…Arnoaldus" succeeded him as bishop, the alleged senatorial and Merovingian ancestry appearing to provide the basis for the 9th century genealogy although the latter assigns the descent to what appears to be a different Arnold. The Gesta Episcoporum Mettensis makes no family connection between Arnulf and his predecessor bishops. Another genealogy from the 8th/9th century names "Buotgisus" as father of "Arnulfum…episcopum urbis Metensium", although the editor of the Monumenta Germaniæ in which this is published cites another source which names "Burtgisus, qui a multis cognominatur Arnoaldus" although the dating of the latter is unclear.
Arnulf entered the service of Theodebert King of Austrasia, becoming intendant of the royal domains. Together with Warnachar, maior domus of the palace of Burgundy, he helped King Clotaire II defeat King Sigebert II and the latter's great-grandmother Queen Brunechildis in 613. Sigeberto's Vita Landiberto episcopi Traiectensis names "Pippinus…principes Francorum…paterni avi eius Arnulfi", specifying that he was "primo maior domus regis post Mettensis episcopus", although no other document has yet been identified which indicates that Arnulf held the position of maior domus in Austrasia. He was elected Bishop of Metz, dated to : the Vita Sancti Arnulfi records that "Arnulfum domesticum adque consiliarium regis" was appointed as bishop of Metz. Arnulf retired to the monastery of Remiremont, Vosges, dated to . A charter dated 20 Feb 691 of "Pippinus filius Ansegisili quondam necnon…matrone mea Plectrudis" donating property to the church of St Arnulf at Metz specifies that "domnus et avus noster Arnulphus" was buried in the church. A list of bishops of Metz records "Arnulfus" as 29th bishop, that he held the position for 10 years, and that he died "XVII Kal Sep". Sigebert's late 11th century Chronica records the death of "Sanctus Arnulfus ex maiore domus Mettensium episcopus, et ex episcopo solitarius" in 640. The Vita Sancti Arnulfi records that Arnulf married "inclitam et nobilissimam...puellam" but gives no further details about her. 1