For a short time he supported his other uncle, Mieszko III the Old, but later decided to reconcile with Casimir II, who after Leszek's death inherited his duchy.

[5] In the past historians were unsure which wife of Bolesław IV the Curly was the mother of Leszek: the first, Wierzchosława, daughter of St. Vsevolod, Prince of Novgorod and Pskov, or the second, Maria.

He based his argument on a document from 26 April 1177, in which Leszek was mentioned as taking the last place among Polish princes, after Mieszko "junior" (Misico iunior dux).

[10] Based on the tympanum from Ołbin made in 1172, historian Kazimierz Jasiński supposed that at that time Leszek was around 10 years old, and therefore he would have been born around 1162.

High Duke Bolesław IV, reportedly devastated by his first-born son's death, died one year later (5 January 1173).

The overlordship of Poland, which included the control over Kraków and Gniezno, was taken by the eldest surviving brother of Bolesław IV, Mieszko III the Old.

[4][12] When in 1177 Casimir II became prince of Kraków, he nominated the magnate Żyron as a guard for Leszek, who suffered from extremely poor health.

[4] According to the 18th-century Russian historian Vasily Tatishchev, based on older historiography, Leszek fought against Prince Volodar of Minsk, who in 1180 captured Brest, which belonged to his brother-in-law, Vasilko Iaropolkovich.

Information provided by Kadłubek supported the theory that he died unmarried and childless[20] According to Vasily Tatishchev, 18th-century Russian historian, citing an unknown Polotsk's Latopis, Vasilko Yaropolkovich, Prince of Drohiczyn, was married to daughter of Leszek, duke of Masovia.

Older historiography considered this information as reliable, correcting early a crucial fact, namely that Vasilko married a sister, not the daughter, of Leszek.

Some Polish historians (first Henryk Rutkowski) considered that after Leszek's death, Mieszko III the Old took control over the western part of Kuyavia.