After death of his father in 1315, Casimir I gained the southwestern part of the duchy, centered around the town of Cieszyn. His marriage to Euphemia of Czersk founded a tradition of relationships of Cieszyn Piasts with Masovian Piasts. Casimir I initially had good relations with Władysław I the Elbow-high, who became the King of Poland in 1320. But when during 1321–1324 Lithuanian forces supporting Elbow-high plundered Cieszyn, Casimir I broke with the Polish King. He then became a closer ally of John of Luxembourg, King of Bohemia and swore homage to him on 8 February 1327 in Opava; in exchange for his submission, Casimir I received the promise of inheritance of Oświęcim. Fifteen days later, on 23 February, he received Cieszyn as a hereditary possession, maintaining a large internal sovereignty. Casimir I wanted to maintain the territorial unity of his Duchy, so he chose only one of his sons to be his successor — firstly the eldest, Władysław, and after his early death in 1355 the third, but the only one who remained outside the Church, Przemysław Noszak — and the others were destined for a Church career. In 1337 he acquired the districts of Siewierz and Czeladź from the Dukes of Bytom by 720 fines; also, he took the town of Namysłów from the Dukes of Brzeg, as a part of the dowry of his daughter Anna. When Duke Leszek of Racibórz died in 1336, Casimir I tried unsuccessfully to obtain his lands. King John of Bohemia granted this land to Duke Nicholas II of Opava. In 1355, after the death of Bolesław, Duke of Koźle-Bytom without male issue, Casimir I entered a conflict over his inheritance with Konrad I of Oleśnica. The dispute was only resolved in 1357 : the Duke of Cieszyn took possession of half of Bytom and Gliwice, Toszek and Pyskowice. In the internal politics, Casimir I supported the economic development of his Duchy, mostly towns. He also surrounded Cieszyn with walls and by 1320 granted the town of Bielsko with German laws. Casimir I died in 1358 and was buried in the Dominican church of Cieszyn.
Marriage and issue
By 1321, Casimir I married Euphemia, daughter of Duke Trojden I of Czersk-Warsaw. They had nine children:
