He was the eldest son of Duke Janusz I of Warsaw and Danutė of Lithuania, a daughter of Kęstutis.
In connection with his preparation to take power in Masovia, Janusz was sent to Kraków, where he resided at the court of King Władysław II Jagiełło.
23 March 1467), a daughter of Spytek of Melsztyn, who was killed in fight with Tatars in the Battle of the Vorskla River on 12 or 16[2] August 1399.
The main reason for the marriage between with a Piast prince with a minor noblewoman was the fortune left by her father.
Katharina's dowry was secured by Janusz with some properties located in the towns of Różan and Nur.