He was the second child (but fourth-born son) of Mieszko III the Old, Duke of Greater Poland and from 1173 High Duke of Poland, due to his marriage to his second wife Eudoxia, daughter of Grand Prince Iziaslav II of Kiev.

In 1184 and thanks to the wise diplomatic affairs of his father, Mieszko the Younger was appointed by the sickly Duke Leszek Governor of the Masovian-Kuyavian principality, and with this, apparently the right of succession after his death.

However, the arbitrary and cruel government of Mieszko III, caused Leszek to reassume his alliance with Casimir II the Just, who was appointed his sole heir.

At this point, Mieszko III decided to give the district of Kalisz to his youngest son and namesake, as his own duchy.

He was buried in the crypt founded by his father in the Cathedral of St. Paul the Apostle in Kalisz.