In 1254, Ciechanów is mentioned as the seat of a castellany (Rethiborius Castellanus de Techanow (Racibor, Kasztelan Ciechanowa)). In 1400 Janusz I of Czersk granted Ciechanów town privileges. The area eventually become a separate duchy with Casimir I of Warsaw using the title "dominus et heres lub dominus et princeps Ciechanoviensis." By the 14th-15th centuries, the town's population reached 5,000. In the early 20th century, there were just under 12,000 inhabitants.
Before World War II, it was home to a large Jewish community but during the Nazi occupation, in the winter of 1942, the majority of the Jewish community were transported to the Red Forest (Czerwony Bór) north-east of town and murdered by gunfire. During the war many Polish Jews and resistance fighters were executed by the Germans in the castle. The town was annexed by Nazi Germany in 1939 and known as Zichenau in German. It was the capital of Regierungsbezirk Zichenau, a subdivision of the Province of East Prussia. The territory was restored to Poland after the war.