Different years of establishment of the town have been attributed by a number of historians since the 16th century. The Diocese of Włocławek (Latin: Vladislaviensis) in Kuyavia, near Płock, was first mentioned in a papal bull issued by Pope Eugene III in 1148. The first bishop of Włocławek was Warner, whose name appears in the 1148 bull, followed by Onoldius from Italy. At some time the diocese was recorded as Vladislaviensis et Pomeraniae ("Włocławek and Pomerania"). The name Vladislaviensis and the time frame point to Władysław II the Exile or his grandfather Władysław I Herman, as well as to Vladislav II of Bohemia, as possible name givers. Under the Nazi occupation during World War II, Włocławek was renamed to Leslau.
Wloclawek had quite a flourishing, sizable Jewish population for many, many hundreds of years. After the Nazi occupation in 1939, it became the first town in Europe requiring yellow Jewish Stars of David to be worn on the backs of the clothes of all Jews, only one obvious part of their persecution. Soon this star became required for all Jews in German occupied Europe. Many Wloclawek Jews died from Nazi fists or guns, starvation or illness after being confined in the Lodz Ghetto. Others were gassed upon arrival at the Chelmno death camp.There is little, if any trace of their once rich and lively community today.