Joscelin lived in the Kingdom of Jerusalem, and managed to gather enough land around Acre to set up the Seigneurie of Joscelin. His sister, Agnes of Courtenay, was the wife of King Amalric I and mother of Baldwin IV. In 1164 Joscelin was taken captive by Nur ad-Din, and remained a prisoner until 1176 when Agnes paid his ransom. She then made him seneschal of Jerusalem. He was therefore allied with the inner circle of the royal family in Jerusalem, which was opposed by the lesser nobles led by Raymond III, count of Tripoli.
In 1180 Joscelin became an ambassador to the Byzantine Empire, replacing William of Tyre, who had lost influence in the royal court. In 1184 he became guardian of the young Baldwin V while Raymond III was regent, thus allying himself with the lesser nobles rather than the royal family; Agnes had probably died by this point, and although Joscelin was Baldwin V's great uncle, Raymond's party of nobles held the most power. However, when Baldwin died in 1186 Joscelin switched sides again, barring Raymond from the funeral.
Joscelin died in 1200. His seigneurie was bought by Hermann of Salza, the master of the Teutonic Knights, in 1220.