Arnulf was the son of margrave Baldwin II of Flanders and Ælfthryth of Wessex, daughter of Alfred the Great. Through his mother he was a descendant of the Anglo-Saxon kings of England, and through his father, a descendant of Charlemagne. Presumably Arnulf was named after Saint Arnulf of Metz, a progenitor of the Carolingian dynasty.
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. However, in 933 Adeloft died, and Arnulf took the countship of Boulogne for himself, but later conveyed it to his nephew, Arnulf II.
Arnulf I greatly expanded Flemish rule to the south, taking all or part of Artois, Ponthieu, Amiens, and Ostrevent. He exploited the conflicts between Charles the Simple and Robert I of France, and later those between Louis IV and his barons.
In his southern expansion Arnulf inevitably had conflict with the Normans, who were trying to secure their northern frontier. This led to the 942 murder of the Duke of Normandy, William Longsword, at the hands of Arnulf's men. 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.
He was buried in the Church of Saint-Pierre de Gand in Ghent.