His parents married in 1399, while Stephen first appears in historical records in late 1408 as endorser of his father's donation to the noble brothers Vukić and Juraj Radivojević for their loyal service.
By 22 July 1417, Stephen reconciled with his father and, with his stepmother, endorsed another Ostoja's donation to the Radivojević brothers, who had helped suppress a rebellion by Zachlumians.
Kaboga urged him to conquer the Ragusan city of Ston, claiming it was not strongly fortified and that the peninsula of Pelješac was not protected.
Stephen claimed the Hrvatinić house in Ragusa, as well as the coast surrounding Slano and all the "royal goods", but eventually relented.
After a council with his mother and the nobility in the town of Zvečaj in March 1419, Stephen confirmed the privileges granted to Ragusa by his predecessors.