As an adult, John Csetneki entered the service of Elizabeth Szilágyi, who financially supported his education and advancement. In this capacity, he served as
ispán of Bereg County and castellan of Munkács from 1474 to 1482; the area belonged to Elizabeth's royal appanage since 1461. John held both titles together with
John Móré from 1474 to 1475, and, thereafter, with
Ladislaus Fornosi from 1476 onwards. During his eight years of service as
ispán and castellan, John received valuable gifts and treasures from his lady Elizabeth Szilágyi, including a silver-plated sword, a silver bowl, and two pearly and wild leather headgear. According to a complaint from 1489, John Csetneki and John Móré made an act of domination, abusing their positions, in 1475. John was admitted as an honorary member to the
Order of Saint Paul the First Hermit in
Buda in 1478 because of his frequent donations to the monastic order. Following the death of Matthias Corvinus in 1490, John Csetneki supported the claim of
John I Albert for the throne during the
War of the Hungarian Succession. Alongside several other families in Upper Hungary, he joined his cause when the future Polish monarch invaded the region in the autumn of 1490. After a peace treaty with his elder brother
Vladislaus II, John Albert renounced his claim to Hungary in exchange for the
Duchy of Głogów and the suzerainty over half of
Silesia on 20 February 1491. In the next month, Vladislaus granted pardon to the partisans of his brother, including John Csetneki, who was able to attend the
Diet of Hungary in Buda as an assessor (a representative of the
lower nobility) for Gömör County in the spring of 1492. In the 1490s, John served as the king's loyalist. When the castellan of
Murány (present-day Muráň, Slovakia) attacked and plundered his nearby estates in 1499, he was acting on behalf of the king somewhere else. John complained that the clash occurred as an instigation of
Palatine Stephen Zápolya. The Csetnekis organized a counterattack against the Bebek family – Zápolya's allies – thereafter. Nevertheless, John had to experience the dissolution of the Csetnek lordship by the end of the 15th century, due to pressure of the powerful Zápolya family, who gradually obtained the possessions of the Bebek family too. According to Hungarian historian József Fógel, John functioned as a royal councillor in 1503. In that year, he was engaged in commercial activities with the Jewish merchant Mendel family of Buda. As one of the four representatives of Gömör County, he participated in the Diet of Hungary in October 1505. Vladislaus II summoned him to Buda to participate in the negotiations with German king
Maximilian I to conclude the
Habsburg–
Jagiellonian mutual-succession treaty in 1506. John was again referred to as among the councilors and jurors of the royal council in 1507. ==Personal life==