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.

Arnulf also made Bruges the center of his administration, contributing to the rise of the town as a major trading hub.

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