During the First Mongol invasion of Hungary, Nicholas participated in the disastrous Battle of Mohi on 11 April 1241, from where fled together with king Béla IV of Hungary to the Dalmatian coast. According to the king's donation letter (1249), Nicholas was sent to diplomatic missions in several unspecified cases in the following years. Belonging to the household of the child prince and heir Duke Stephen, Nicholas served as "ispán" of Dubica County from 1244 to 1249, which had laid in Lower Slavonia. In this capacity, he fought against the heretics in Bosnia between 1244 and 1246. Nicholas was also present at the Battle of the Leitha River on 15 June 1246, where the Hungarians were defeated but their enemy, Frederick the Quarrelsome was killed. In 1249, Nicholas was styled as treasurer in the ducal court of Stephen.