The son of Kazimierz I and Eufemia of Mazovia. Like his elder brothers, Bolesław and Jan, he chose to enter the clergy, joining the Order of the Knights Hospitallers. In 1361 he became a commander of the order’s monastery in Oleśnica. About 1372 he received the title of Provincial of the Knights Hospitallers in Austria, Bohemia, Moravia, Poland and Styria, which he retained until his death. Siemowit's career was possible owing to the support of Charles IV, who in that way repaid Przemysław Noszak, Siemowit's brother, for Noszak's faithful service. 

(K. Jasiński, Rodowód, pp. 145-146, I. Panic, Księstwo, p. 52)