Background
He was the second son of Konrad III the Red and his third wife Anna, a daughter of Mikolaj Radziwiłł the Old, Voivod of Vilnius and the first Grand Chancellor of Lithuania.
He was the second son of Konrad III the Red and his third wife Anna, a daughter of Mikolaj Radziwiłł the Old, Voivod of Vilnius and the first Grand Chancellor of Lithuania.
Despite this, Anna Radziwiłł retained the real power in Masovia until her death in 1522. Despite being the co-ruler of their domains, Janusz III didn"t participate in the government until Stanisław"s death on 8 August 1524, when he finally began his sole government. His dissolute lifestyle problably contributed with is early death, who took place in the night of 9 to 10 March 1526.
He was buried at Saint John"s Archcathedral, Warsaw.
With his death, the male line of Masovian Piasts, originating with Siemowit III became extinct. The death of both brothers caused unrest, and accusations that they were murdered became widespread.
Eventually, King Sigismund I himself looked into the matter, and concluded that there was no foul play. According to January Długosz, the real cause of the death of both princes could be an inherited disease of the Masovian princes: tuberculosis.
A contemporary historian, Marcin Bielski, suggested that both brothers died due to alcohol poisoning.
Soon after Janusz III"s death the Duchy of Masovia was incorporated into the kingdom of Poland, despite resistance from some of the Masovian nobility who tried to retain their independence and argued that the Duchy should be inherited by the female relatives (such as Anna or Sophia of Masovia). The Polish king refused to recognize their demands, and stood by the agreements that made him the heir to the Duchy, reuniting it with Poland. The Duchy, which would become a significant asset of the Polish Jagiellon dynasty, would retain some autonomy until 1576.
He is one of the characters on the famous painting by January Matejko, Prussian Homage.