William succeeded Rollo (who was still alive) in 927 and, early in his reign, faced a rebellion from Normans who felt he had become too Gallicised and too soft. According to Orderic Vitalis, the leader was Riouf of Evreux. At the time of this rebellion William sent his pregnant wife Sprota to Fécamp where their son Richard was born.

In 933, William I Longsword recognized Raoul as King of Western Francia, who was struggling to assert his authority in Northern France. In turn Raoul gave him lordship over much of the lands of the Bretons including Avranches and the Cotentin. Resistance to the Normans was led by Alan Wrybeard, Duke of Brittany and Count Berenger of Rennes but ended shortly with Alan fleeing to England and Beranger seeking reconciliation.

In 935, William contracted a marriage between his sister Adela (Gerloc was her Norse name) and William, count of Poitou with the approval of Hugh the Great. At the same time William married Luitgarde, daughter of count Herbert II of Vermandois whose dowry gave him the lands of Longueville, Coudres and Illiers l'Eveque. In addition to supporting king Raoul, he was now a loyal ally of his father-in-law, Herbert II, both of whom his father Rollo had opposed.

William Longsword attacked Flanders in 939 and Arnulf I, Count of Flanders, and Louis IV, King of France, retaliated by attacking Normandy. Arnulf captured the castle of Montreuil-sur-Mer expelling Herluin, Count of Ponthieu. Herluin and William Longsword cooperated to retake the castle. William was excommunicated for his actions in attacking and destroying several estates belonging to Arnulf.

William pledged his loyalty to King Louis IV when they met in 940 and, in return, he was confirmed in lands that had been given to his father, Rollo. Almost three years later, on 17 December 942 at Picquigny on the Somme, William Longsword was ambushed and killed by followers of Arnulf while at a peace conference to settle their differences (wikipedia)