Istvánffy returned home in 1556 and presumably became a soldier of Nikola Šubić Zrinski (), the hero of Szigetvár. Between 1558 and 1559 he functioned as the secretary of Olahus, who served as Chancellor of Hungary from 1543. Istvánffy became an official of the Chancellery after 1559. Olahus was appointed Royal Governor of Hungary in 1562. He died in 1568.