After death of his father in 1315, Casimir I gained the southwestern part of the duchy, centered around the town of Cieszyn.

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.

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.

Casimir I died in 1358 (although there are some indications that he lived until 1360, but this is refuted by the majority of historians) and was buried in the Dominican church of Cieszyn.