The son of the first Prince of Cieszyn, Mieszko. The name of his mother is unknown. After his father's death circa 1315 and the subsequent division of the Duchy of Cieszyn, he received the south-western part, the former Cieszyn castellany, which from that time became the actual Duchy of Cieszyn, the basis of what was to become Cieszyn Silesia. His elder brother, Władysław, inherited the north-eastern part of what had been the Duchy of Cieszyn and was to become the new Duchy of Oświęcim.
      In 1321 Kazimierz married Princess Eufemia, often referred to as Ofka (* c.1310 + after January 1374), the daughter of Trojden I, the Prince of Sochaczew and Czersk, and Maria, the daughter of Jerzy I, the King of Halicz, establishing the tradition of marriages between Cieszyn and Mazovian Piasts. Owing to the degree of consanguinity between them a papal dispensation was necessary. Eufemia was most probably buried by her husband's side in the Dominican church in Cieszyn.
      Kazimierz and Eufemia had nine children. Of these there were five sons; Przemysław I Noszak, who inherited the Duchy of Cieszyn from his father; Siemowit, a prior of the Knights Hospitallers; Jan, a cleric of the Wrocław diocese; Bolesław, a chancellor of Emperor Charles IV and Władysław, a judge at the court of Emperor Charles IV. There were four daughters; Agnieszka, who married the Prince of Oleśnica, Konrad II; Anna who married the Prince of Legnica, Wacław I; Elżbieta, a Cistercian nun in Trzebnica and Jolanta Helena, a nun of St. Clare in Cracow.
      Kazimierz was initially on good terms with Prince Władysław Łokietek, who was crowned King of Poland in 1320. When in 1326 a Lithuanian army coming to provide reinforcements for Władysław also invaded the Duchy of Oświęcim it triggered a conflict between the Prince of Cieszyn and Władysław. This made it easier for Kazimierz to decide to support John of Luxembourg and consequently to swear his allegiance to the King of Bohemia on 8 February 1327 in Opava. On the strength of a document of 23 February 1327 he received the Duchy of Cieszyn as a hereditament, retaining far-reaching domestic sovereignty.
      Kazimierz made efforts to maintain the territorial integrity of his duchy. For that reason he finally bequeathed the throne in its entirety to Przemysław Noszak. With regard to his other sons he planned careers in the clergy or sought advancement for them at the royal court in Prague. At the same time he made efforts to extend the territory of his dominion. In 1337 he bought the Siewierz lands from the Princes of Bytom; Władysław and Kazimierz. He took the Namysłów lands in lien from the Prince of Brzeg, Bolesław III Szczodry alias Rozrzutny (Boleslaus III the Generous alias the Spendthrift). He also took part in the dispute over the legacy of the princes of Koźle and Bytom – concluded definitively only after his death. As a result of this Prince Przemysław Noszak gained possession of half of Bytom and the Duchy of Bytom. Historians maintain that it was he who rebuilt the castle in Cieszyn and built the town walls. Following in his father's footsteps he helped the economy in the Duchy of Cieszyn to flourish, supporting the growth of towns. For example between 1316 and 1327 he established Bielsko according to 'German Law'.

(K. Jasiński, Rodowód, pp. 120-122, O. Balzer, Genealogia, p. 452, I. Panic, Księstwo cieszyńskie, pp. 43-45, id., Jeszcze o dacie, p. 81, id., Jeszcze o przejęciu Siewierza, pp. 88-89)