Arnulf was the son of margrave Baldwin II of Flanders and Ælfthryth of Wessex, daughter of Alfred the Great.

At the death of their father in 918, Arnulf became Count of Flanders while his brother Adeloft or Adelolf succeeded to the County of Boulogne.

[8] Arnulf I greatly expanded Flemish rule to the south, taking all or part of Artois, Ponthieu, Amiens, and Ostrevent.

[10] The Viking threat was receding during the later years of Arnulf's life, and he turned his attentions to the reform of the Flemish government.

When Dunstan, abbot of Glastonbury, fled into exile, Arnulf received him with honour and lodged him in the Abbey of Mont Blandin, near Ghent.