Like most of the prelates of his time he took a prominent part in the struggle against the Northmen, by whom he and his brother Louis were taken prisoners (858), and he was released only after paying a heavy ransom (Prudentii Trecensis episcopi Annales, ann.
When the attack finally came (885), the defence of the city was entrusted to him and to Odo, Count of Paris, and Hugh the Abbot.
[1] The city was attacked on 26 November and the struggle for the possession of the bridge (now the Pont-au-Change) lasted for two days; but Joscelin repaired the destruction of the wooden tower overnight, and the Vikings were obliged to give up the attempt to take the city by storm.
The Siege of Paris lasted for about a year longer, while the emperor Charles the Fat was in Italy.
Joscelin died soon after the preliminaries of the peace had been agreed on, worn out by his exertions, or killed by a pestilence which raged in the city.