Leszek of Masovia was the second Duke of Masovia and Kuyavia from 1173 until his death.
Background
He was the second but only surviving son of Bolesław IV the Curly, High Duke of Poland by his first wife Viacheslava, daughter of Saint Vsevolod, Prince of Novgorod and Pskov. Once it was thought that his mother was Maria, Bolesław IV"s second wife, a fact which was shown in several web sources. However, after the discovery of coins where Leszek called himself son of Bolesław and Anastasia (the Greek or Latin equivalent of Viacheslava"s name), this theory proved to be inaccurate.
He was named either thanks to the courtly tradition preserved by Gallus Anonymus in his Cronicae Polonorum or after the eldest full-brother of his father, Leszek, who died young around 1131.
Career
High Duke Bolesław IV (reportedly devastated by his first-born son"s death) died one year later (3 April 1173), leaving Masovia and Kuyavia to Leszek, at the age of eleven or less. The overlordship of Poland (who included the control over Krakow and Gniezno), was taken by the eldest surviving brother of Bolesław IV, Mieszko III the Old. According to the Bishop of Krakow and chronicler Wincenty Kadłubek, the young Duke of Masovia suffered of an extremely poor health and even after he formally took the government of his domains in 1177, the effective rule was handed by the magnate Żyron.
Since he take the control over his domains, Leszek"s policy was based on a close cooperation with Casimir II the Just, whose interests in Masovia (after he took the title of High Duke in 1177) were represented by the magnate Żyron.
A manifestation of good relations with Casimir II was the participation of Leszek in the war against Prince Volodar of Minsk, who in 1180 captured Brest, who belonged to Leszek"s brother-in-law, Vasilko Iaropolkovich. After a long and exhausting war, Brest ultimately wasn"t recovered, and Vasilko gave all the rights over this land to Leszek (the region was annexed to Poland only by Casimir II in 1191, after he deposed Mieszko III and took the title of High Duke).
In 1184 Leszek unexpectedly changed his dynastic politics. Probably the real purpose of the appointment of Mieszko the Younger as governor of Masovia was prepared him as Leszek"s successor after his death.
The main reason of this attitude maybe were the rude and harsh proceedings of the Greater Poland Duke.
Leszek died unmarried and childless in 1186. He was probably buried in the Płock Cathedral. The High Duke only could take Masovia, but thanks to this inheritance, he became in the most powerful ruler of Poland.