Mieszko I was the first historic king of Poland which was actually recorded in writing. The noted historian Gall Anonim calls Mieszko I the first of the line of the Piasts, the legendary start of that dynasty. The year of Mieszko's birth is not exactly known, neither is the exact date of his reign, however it is certain that he was king before 963. His reign started with a small territory between the Oder and Wisła rivers, and subsequently left it a large kingdom which before long would be a great Central European political power. Mieszko knew that before him lay a very hard task: to consolidate the care-free attitude of a young nation, and to provide a national interest. He had to defend the young nation against the attacks of the Germans and also to gather all people of Polish blood to defend themselves. He also wanted to use this self-identity to consolidate a large Slavic region in Europe.

To help in his gathering of the Polish people, Mieszko, and all his court and servants adopted Christianity in the year 966. Christianity was not adopted from the Germans, but rather from the Czech people by Mieszko's marriage to Dobrawa the daughter of Bolesław I in the year 965. In this manner Poland was unified within the Slavic brotherhood and gained an understanding with the Papacy which helped the Polish culture and civilization in adopting views of the west. The adoption of Christianity also helped strengthen Mieszko's reign.

Throughout all of Mieszko's reign, he fought off attacks by neighboring German feudal lords and the reigning Caesar. In the year 972 he granted the Germans the lands beyond the river Warta. To ensure lasting peace with the Germans he married Ode, the daughter of the noble Theodore after the death of Dobrowa. Further in the year 986 he gave the lands west of the Oder river to the Germans and before his death gave back lands under the protection of the Papacy.

Mieszko wasn't able to stop the wars with the Czechs and the Kiev Russians and in the year 981 lost the lands near the rivers San and Bug. Thanks to the marriage of Mieszko's sister and daughter relations were solidified between the Hungarians and Swedes who had large enough armies to help repel more German attacks. He also gained the lands of Little Poland and Silesia and gradually acquired parts of Pomerania.

A Jew from Muslim Spain Ibrahim Ibn Jakub documented much of Mieszko's reign and history along with other Slavic countries. Mieszko I died in the year 992 and is likely buried under the cathedral in Poznań.