Mikołajki is an old Masurian church town first documented as Nickelsdorf (Sankt Niklas) in 1444 and Niklasdorf in 1493. The name refers to Saint Nicholas ("Mikołaj" in Polish). Early on, it was part of the State of the Teutonic Order. In 1454 Polish King Casimir IV Jagiellon incorporated the region to the Kingdom of Poland upon the request of the anti-Teutonic Prussian Confederation, and the Thirteen Years' War broke out. After the war ended in 1466 it was part of Poland, as a fiefdom held by the Teutonic Knights and, from 1525 on, the Duchy of Prussia, founded as a vassal state of Poland. Similar to all of Masuria, it was mainly inhabited by Poles, hailing from nearby Masovia. In 1539 the inhabitants of the settlement were almost entirely Poles, who called it by its Polish name "Mikołajki". First Protestant pastors were mentioned in 1552. The settlement, now within the Kingdom of Prussia, grew during the 18th century, receiving its town privileges as Nikolaiken in 1726. Because of its location on Śniardwy, the fishery of Nikolaiken ensured continued prosperity; the whitefish of the region were especially popular throughout the province of East Prussia.