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, probably on the brink of his father's death. It's however also possible that he was born shortly after, and in consequence was posthumous. Maybe this was the reason that in the Bolesław III's Testament, he was omitted and left without any land.
During his first years, Casimir and his sister Agnes (born in 1137) lived with their mother Salome in her widow land of Łęczyca. There, the young prince remained far away from the struggles of his brothers Bolesław IV the Curly and Mieszko III the Old with their older half-brother High Duke Władysław II, who tried to reunite all Poland under his rule and in 1146 was finally expelled.
Salomea of Berg had died in 1144. Casimir and Agnes were cared by their elder brother Bolesław IV, who had assumed the high ducal title. Although under his tutelage the young prince could feel safe, he had no guarantee to receive part of the paternal inheritance in the future. When in 1154 he reached the proper age (according to the standards of that time) to take control over the lands of the family, he remained with nothing. Even worse, three years later (1157) his fate was decided in the successfully Polish campaign of the Emperor Frederick Barbarossa, who came to the aid of Władysław II and his sons. As a part of the treaty Bolesław 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.
The fate of Casimir at the Imperial court is unknown. He returned to Poland certainly before 21 May 1161, because that day he is mentioned in a document with two of his brothers, Bolesław IV and Henry of Sandomierz. So far, Casimir had not received any lands from his elder brothers.
The situation changed in 1166, when his brother Henry was killed in battle during a Prussian Crusade; without issue, in his will he named Casimir the only heir of his Lesser Polish Duchy of Sandomierz. However, High Duke Bolesław IV decided to divide the duchy into three parts: the largest (who included the capital, Sandomierz) to him; the second (without any name) to Mieszko III and only the third part, the small district of Wiślica, was given to Casimir.
Angry and disappointed with the decision of the High Duke, Casimir rebelled against him, with the support of by his brother Duke Mieszko III the Old, the magnate Jaksa of Miechów and Sviatoslav, son of Piotr Włostowic, as well as Archbishop Jan of Gniezno and Bishop Gedko of Kraków; also, almost all Lesser Poland was on his side. The quick actions of Bolesław finally stopped the rebellion. At the end, Casimir was only able to retain Wiślica. In 1172, Mieszko III the Old rebelled against the High Duke, and tried to persuade his younger brother to join him. For unknown reasons, Casimir refused to participate.
Bolesław IV died in 1173 and according to the principle of agnatic seniority he was succeeded by his brother Mieszko III the Old as High Duke. He decided to give the rest of the Sandomierz duchy to Casimir, then his only surviving brother, who finally could assume the ducal title, which the late High Duke had illegally usurped.
The strong and dictatorial rule of the new High Duke caused a deep disaffection among the Lesser Polish nobility. This time the new revolt prepared in 1177 had a real chance of victory. The rebellion, apart of the magnates, counted with the support of Gedko, Bishop of Kraków, Mieszko's eldest son Odon, the son of former High Duke Władysław II, Duke Bolesław I the Tall of Silesia and Casimir. The reasons about his inclusion in the revolt, after being reconciled with Mieszko, are unknown.
The battle for the supreme power had a quite strange course: Mieszko, completely surprised by the rebels in his Duchy of Greater Poland, withdrew to Poznań, where he stayed for almost two years going heavy fighting with his son Odon. Finally, he was defeated and was forced to escape. 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. After a successfully action in Silesia, he marched to Kraków, who was quickly conquered. Casimir, now Duke of Kraków, decided to conclude a treaty under which Bolesław the Tall obtain the full authority over Lower Silesia at Wrocław, in return Casimir granted to the then deposed Mieszko Tanglefoot the Lesser Polish districts of Bytom, Oświęcim and Pszczyna as a gift for Casimir's godson and namesake: Casimir I of Opole, the only son of Mieszko Tanglefoot.
The 1177 rebellion against High Duke Mieszko the Old ended in a full success to Casimir, who not only had 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 Tanglefoot and Konrad Spindleshanks as well as Bolesław's son Jarosław of Opole), Greater Poland (ruled by Odon), 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). On the Baltic coast, Pomerelia (Gdańsk Pomerania) was ruled by Duke Sambor I as a Polish vassal.
However, Mieszko the Old worked intensively for his return, at first in Bohemia, later in Germany and in the Duchy of Pomerania. To achieve his ambitions to give the hereditary right to the throne at Kraków (and with this the Seniorate) to his descendants, Casimir called an assembly of Polish nobles at Łęczyca in 1180. He granted privileges to both the nobility and the Church, lifting a tax on the profits of the clergy and relinquishing his rights over the lands of deceased bishops. By these acts, he won the acceptance of the principle of hereditary succession to Kraków, though it still would take more than a century to restore the Polish kingship.
In less than a year after the Łęczyca assembly however, in the first half of 1181, Mieszko the Old 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). At the same time, Duke Leszek of Masovia decided to escape from the influence of Casimir. He named Mieszko the Old's son Mieszko the Younger governor of Masovia and Kuyavia, and with this, made a tacit promise of succession over these lands.
For unknown reasons, Casimir didn't react to these events and only decided to secure his authority over Lesser Poland. A diplomatic meeting occurred in 1184 at the court of the Emperor Frederick Barbarossa where Casimir, in order to reject the actions of Mieszko the Old and to retain the power over all Poland, swore allegiance to Barbarossa and paid him a large tribute.
The most important issue during the reign of Casimir beside the conflict with his brother Mieszko was the issue of the diplomatic policies towards the neighbouring Russian principalities in the east. The first task before which he became in High Duke was his attempts to create bonds with the Rurik Grand Princes at Kiev, who were strongly associated with the previous High Dukes through marriages with Kievan princesses (Bolesław the Curly with Viacheslava of Novgorod and Mieszko the Old with Eudoxia of Kiev). For this purpose, in November 1178 Casimir arranged the marriage of his daughter Maria with Prince Vsevolod IV of Kiev.
His first major intervention in the Russian affairs occurred in 1180, when in the beginning of the dispute between Prince Vasilko, Prince of Shumsk and Dorohychyn (son-in-law of late Bolesław the Curly) and Leszek of Masovia for the region of Volhynia at Volodymyr-Volynsky, the High Duke support of the former. The war ended with the success of Casimir, who conquered Volodymyr and Brest, while Vasylko held his ground at Drohiczyn.
The war had not definitively settled about the matter of the property of Brest, which was granted as a fief to Prince Sviatoslav, Vasilko's cousin and also Casimir's nephew (son of his sister Agnes). In 1182 another revolt broke out against Svyatoslav's rule, but thanks to Casimir's intervention, he was restored in the throne. Nevertless, shortly after Casimir saw that the situation was unstable, he finally decided to give the power to Sviatoslav's brother, Roman.
In 1187 Prince Yaroslav Osmomysl of Halych died, whereafter a long struggle for his succession began. Initially, the authority over the principality was taken by his younger son, Oleg, but soon he was murdered by the boyards and Halych was taken by the eldest son of Yaroslav, Vladimirko. Vladimirko's reign was also far from stability, a situation used by Prince Roman of Brest, who, with the help of his uncle Casimir, deposed him and took full control over Halych.
Defeated Vladimirko fled to the Kingdom of Hungary under the protection of King Béla III (his relative; Vladimirko's paternal grandmother was a Hungarian princess), who decided to send his army to Halych. Roman escaped to Kraków and Vladimirko, as an act of revenge, invaded Lesser Poland. However, soon Béla III decided to attach Halych to Hungary, and again deposed Vladimirko, who was replaced as Prince of Halych by the King's second son, Andrew. The war continued until two years later, when Casimir followed the instructions of Emperor Frederick Barbarossa, who had decided to help Vladimirko after he had declared himself his subject, and restored his authority over Halych.
In 1186 Duke Leszek of Masovia died. Before his death the sickly duke decided to give all his lands to High Duke Casimir. Though previously, Leszek had promised the inheritance to his elder uncle Mieszko the Old, his dictatorial proceedings had changed his mind and decided in Casimir's favor. Shortly after Leszek's death however, Mieszko occupied the lands of Kuyavia up to the Vistula River, and Casimir only could take possession over Masovia proper. Nevertheless, thanks to the Masovian inheritance, Casimir directly ruled over the major part of Poland.
The involvement of Casimir in the Russian affairs was used in 1191 by Mieszko the Old, who managed to take control over Wawel Castle at Kraków, seizing the high ducal title and the control over the Seniorate Province. Immediately, he declared Kraków an hereditary fief to his own descendants, implementing his son Mieszko the Younger as a governor. The conflict finally ended peacefully, as Casimir – returned from Russia – regained the capital without fight, after Mieszko the Younger had escaped at the side of his father.
Casimir planned to found a University in Kraków and already started to build the building, but his sudden death balked his plans. The present-day Jagiellonian University was not established until 1364 by King Casimir III the Great as the second oldest in Central and Eastern Europe (after the Charles University in Prague).
The last goal of Casimir's reign was at the beginning of 1194, when he organized an expedition against the Baltic Yotvingians. The expedition ended with a full success, and Casimir had a triumphant return to Kraków. After a banquet was held to celebrate his return, Casimir died unexpectedly, on 5 May 1194. Some historians believed that he was poisoned. He was succeeded as High Duke by his eldest surviving son Leszek I the White, who like his father had to face the strong opposition by Mieszko the Old. He was probably buried at Wawel Cathedral.
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 and Regular Canonry of Czerwińsk nad Wisłą and Trzemeszno and the Order of the Knights Hospitaller in Zagość. He also tried to expand the cult of Saint Florian, whose remains were brought to Kraków by Bishop Gedko.
Between 1160-1165 (but no later than 1166), Casimir married with Helena (ca. 1140/42 – ca. 1202/06), daughter of Duke Conrad II of Znojmo, scion of a Moravian cadet branch of the Přemyslid dynasty. They had seven children:
- A daughter (name unknown) (b. before 1167), married between 11 October and 24 December 1178 to Prince Vsevolod IV of Kiev.
- Casimir (ca. 1162 – 2 February or 1 March 1167), named after his father.
- Bolesław (ca. 1168/71 – 16 April 1182/83), probably named after his paternal grandfather Bolesław III Wrymouth, although is possible that in fact was named in honour to his uncle Bolesław IV the Curly. He died accidentally, after falling from a tree. He was probably buried at Wawel Cathedral.
- Odon (1169/84 – died in infancy). He was probably named after either Odon of Poznań or Saint Odo of Cluny.
- Adelaide (ca. 1177/84 – 8 December 1211), foundress of the convent of St. Jakob in Sandomierz.
- Leszek I the White (ca. 1184/85 – 24 November 1227).
- Konrad (ca. 1187/88 – 31 August 1247).