He became ruler over the Polish Seniorate Province at Kraków and thereby High Duke of Poland in 1177; a position he held until his death, though interrupted once by his elder brother and predecessor Mieszko III.

In 1186 Casimir also inherited the Duchy of Masovia from his nephew Leszek, becoming the progenitor of the Masovian branch of the royal Piast dynasty, and great-grandfather of the later Polish king Władysław I the Elbow-high.

Casimir, the sixth but fourth surviving son of Bolesław III Wrymouth, Duke of Poland, by his second wife Salomea, daughter of Count Henry of Berg, was born in 1138, after his father's death but on the same day.

There, the young prince remained far away from the struggles of his brothers Bolesław IV the Curly and Mieszko III with their older half-brother High Duke Władysław II, who tried to reunite all of Poland under his rule (contrary to his late father's testament) and was finally expelled in 1146.

Three years later (1157), his situation worsened as a result of the successful Polish campaign of Emperor Frederick Barbarossa, who came to the aid of Władysław II and his sons.

As a part of the treaty Bolesław IV had to conclude with Barbarossa, Casimir was sent to Germany as a hostage in order to secure the loyalty of his brother to the Emperor.

Angry and disappointed with the decision of the High Duke, Casimir rebelled against him, with the support of his brother Mieszko, the magnate Jaksa of Miechów, Sviatoslav son of Piotr Włostowic, Archbishop Jan of Gniezno, and Bishop Gedko of Kraków.

The battle for new leadership took quite strange course: Mieszko III, completely surprised by the rebels in his Duchy of Greater Poland, withdrew to Poznań, where he stayed for almost two years enduring heavy fighting with his son Odon.

Duke Bolesław the Tall failed to conquer Kraków and the Seniorate Province, as he himself was stuck in an inner-Silesian conflict with his brother Mieszko I Tanglefoot and his own son Jarosław; soon defeated, he asked Casimir for help.

The 1177 rebellion against High Duke Mieszko III was a complete success for Casimir, who not only conquered Kraków (including the districts of Sieradz and Łęczyca) obtaining the high ducal title, but also managed to extend his sovereignty as Polish monarch over Silesia (then divided between the three sons of Władysław II: Bolesław the Tall, Mieszko I Tanglefoot, and Konrad Spindleshanks, as well as Bolesław's son Jarosław of Opole), Greater Poland (ruled by Odon), and Masovia and Kuyavia (ruled by Duke Leszek, then a minor and under the tutelage of his mother and the voivode Żyrona, one of Casimir's followers).

In order to achieve his ambitions and give the hereditary right to the throne at Kraków (and with this the Seniorate Province) to his descendants, Casimir called an assembly of Polish nobles at Łęczyca in 1180.

However, in the first half of 1181 (and less than a year after the Łęczyca assembly), Mieszko III, with the assistance of Duke Sambor's brother Mestwin I of Pomerelia, conquered the eastern Greater Polish lands of Gniezno and Kalisz and managed to persuade his son Odon to submit (according to some historians, Odon then received from his father the Greater Polish lands south of the Obra River).

[a] His first major intervention in Kievan Rus' affairs occurred in 1180, when the High Duke supported Vasylko, Prince of Shumsk and Drohiczyn (and son-in-law of the late Bolesław IV the Curly), and his nephew Leszek of Masovia in a dispute with Vladimir of Minsk for the region of Volhynia at Volodymyr.

However, this war did not definitively settle the matter of the rule at Brest, which had been granted as a fief to Prince Sviatoslav, Vasylko's cousin and Casimir's nephew (stepson of his sister Agnes).

The conflict ended peacefully, as Casimir – upon his return from Russia – regained the capital without a fight, and Mieszko the Younger escaped to the side of his father.

During his reign, Casimir was very generous to the Church, especially with the Cistercians monasteries of Wąchock, Jędrzejów, Koprzywnica and Sulejów; with the Canons of the Holy Sepulchre of Miechów, Regular Canonry of Czerwińsk nad Wisłą and Trzemeszno and the Order of the Knights Hospitaller in Zagość.