Siemowit (also Ziemowit) was, according to the chronicles of Gallus Anonymus, the son of Piast the Wheelwright and Rzepicha.
At first it was part of another Polish fiefdom, the Duchy of Masovia, as in 1388 the king of Poland, Władysław Jagiełło, granted Belz to Siemowit IV, Duke of Masovia, for his recognition of Masovia as a fiefdom of Poland and as a dowry for Siemowit's marriage with Jagiełło's sister, Alexandra.
While Siemowit's son Duke Konrad II (1264–1294) moved his residence to Czersk he and his brother Boleslaus II entered into a long-term conflict over the Polish seniorate with their Kuyavian relatives and the Silesian Piasts, which estranged them from the Piast monarchy.
As the oldest son Siemowit obtained the central part of Masovia, with Rawa Mazowiecka (capital) Sochaczew, Zakroczym, Gostynin, Ciechanów and Wizna.
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As a potential successor Casimir III the Great on the throne of Poland, Siemowit II agreed to waive the rights of Chełmno and Eastern Pomerania.
When Casimir III the Great died in 1370, under agreements concluded with one of his lifetime friends Siemowit III, he freed Siemowit from Polish suzerainty and gave Płock, Wizna, Wyszogród and Zakroczym back to Mazovia.
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Siemowit gave the small region of Warsaw to his brother Casimir, retaining the regions of Czersk, Liw and Rawa.
Duke Siemowit, being ambitious, was very active in politics of the period and thus needed cash to finance his policies so several times he mortgaged some of his domains to the Teutonic Order, including Wizna, Zawkrze and Płońsk.
The next information about Siemowit is found on 19 February 1327, when, together with his brothers Władysław and George, he paid homage to the Bohemian King John of Luxembourg in Opawa.
As a Teutonic Knight, Siemowit was successively named komturem of Oleśnica Mała near Oława since 1360, Prior of Poland, Bohemia, Moravia, Austria, Styria and Carinthia since 1372 and Governor and Treasurer of the Order in Germany since 1384.
Siemowit V (also spelled Ziemowit; 1389 – 16 February 1442) was from 1426-1434 a Prince of Masovia in Rawa, Plock, Sochaczew, Gostynin, Płońsk, Wizna and Belz i.e., hereditary vassal Polish, the breakdown in 1434 goes as follows; prince of Rawa, Gostynin and Sochaczew.
The circumstances around the removal of Siemowit are unknown, and the next notices about him are found only on 19 February 1327, where Władysław, Siemowit and George paid homage to the Bohemian King John of Luxembourg in Opawa.