Leszek was the eldest son of Duke Casimir I of Kuyavia and his second wife, Constance, daughter of Henry II the Pious from the Silesian branch of the Piast dynasty.

However, the colonization policy which he pursued (including the foundation of cities like Nowa Brzeźnica, Lutomiersk, Wolbórz, and Radomsko) and the close cooperation with the Church slowly changed the image of the district.

His government over this land lasted until 1278, when after a meeting at Ląd on 24 August, and thanks to the mediation of Przemysł II of Greater Poland, Leszek returned the duchy to his brother.

The two princes meet for the first time in 1260, on occasion of the expedition against the Kingdom of Bohemia, which was part of a broader action during the Hungarian-Bohemian War for the Babenberg inheritance.

In 1274 Władysław Opolski and Bolesław the Chaste decided to conclude a peace, under which the former gave up his claims to the throne of Kraków in exchange for some territory adjacent to his duchy, stretching as far as the Skawinka river.

In 1265 Leszek married Gryfina (also known as Agrippina), daughter of the Rurikid prince Rostislav Mikhailovich, Ban of Slavonia and Duke of Macsó.

The union was quite unhappy; in 1271 a scandal erupted when she fled to Kraków and publicly accused her husband of impotence, adding that for this reason their marriage had never been consummated.

The reconciliation of the couple took place four years later, due to the intervention of Bolesław the Chaste and his wife, Kinga, Gryfina returned to her husband on 6 August 1275.

His prescription included eating frogs and snakes, because - as was stated in the Rocznik Traski - "the lack of offspring caused a great abomination into the nation".

With the help of Lithuanians, Tartars, and some Russian principalities, Leo invaded Lublin in February 1280, crossed the Vistula, and besieged Sandomierz, which managed to resist.

During 1282-1283, the conflict came to its most dramatic stage, when Bishop Paweł (who fiercely supported Kinga's rights) was captured after a meeting at Łagów and imprisoned in Sieradz.

A final settlement was signed on 30 November 1286 when Leszek agreed to pay Bishop Paweł 3,000 grzywnas as compensation for damages, return his property, and recognize the Bishopric's privileges.

The government of Leszek also garnered opposition from the local knights, which would be surprising given the numerous times that they served the High Duke in his victorious expeditions.

On 3 May 1285, a decisive battle took place in Bogucice, where Leszek II, with the help of the Hungarians, obtained a great victory and forced the rebels to leave the country.

Eventually, Henryk IV Probus became the new high duke in 1289, but his sudden death one year later caused the Seniorate Province to pass to Przemysł II.

Having no legal grounds to reign, Wenceslaus II managed to obtain a document from his aunt Dowager Duchess Gryfina under which she ceded to him the district of Stary Sącz, which she finally received as her dower, with the doubtful inclusion of all of the Seniorate Province.