In 1429, the Grand Duke of Lithuania Vytautas the Great granted Ciechanowiec the settlement Magdeburg rights and its coat of arms.

In the mid-16th century Castellan of Trakai, Piotr Kiszka built a castle on the right bank of the river Nurzec, northeast of the town.

Between 1617–1642, Mikołaj Kiszka ordered to build heavy defensive walls around the fortress, but soon the castle burned down during the Deluge, and the surviving buildings with the newly-built residence for the Ossoliński family were later blown up and destroyed by the Imperial Russian Army during World War I (1915).

In 1736–1739, a brick church of the Holy Trinity and the Sisters of Mercy hospital was built, according to the draft of Warmia's architect Jan Adrian Kluk.

Jan Krzysztof Kluk (1739–1796), the local parish priest, devoted to natural history, became one of the most important Polish naturalists of the Enlightenment.

In the era of the partitions, Ciechanowiec was passed onto Prussia, and after the Congress of Vienna to the Russian Empire.

Jewish family names like Ciechanowiec, Ciechanowiecki, Ciechanowicz, Ciechanowski and Chechanover originated from this town.