In either 964 or 965 (more probably) he married Dubrawka (or Dobrava), a daughter of Boleslaus I, duke of Bohemia. In 978 he married Oda von Haldensleben, daughter of Dietrich (Theoderic) of Haldensleben, count of the North March (965-985), after abducting her from the monastery of Kalbe.
The early career of Mieszko was dominated by fighting with the tribes of Wieletes and Volinians south of the Baltic Sea, and their ally, the Saxon count Wichman. Mieszko was baptised in 966, probably under the influence of his Christian first wife and in order to avoid confrontation with the Holy Roman Empire to the west; he built a church dedicated to Saint George at Gniezno.
At the time of the reign of Mieszko there was no single place serving as the capital instead he built serveral castles around his country. One the most important was Ostrow Lednicki[?] (what is supported by the recent archeological findings (http://www.slavia.org/Lednica)). It was a ring-fort some 460 feet in diameter. Inside his residence, a fine stone palace, the country's first monumental architecture.
He had probably one sister of unknown name, and two brothers: one of them, name unknown, was killed in battle around 964; and the second, named Czcibor, died in the Battle of Cedynia in 972.
His reign began around 962 in Greater Poland (Wielkopolska), Cujavia[?], Masovia and possibly in eastern Pomerania. In the 960s he probably at least partially conquered western Pomerania, and in the 990's he conquered Silesia and Little Poland (Malopolska).
Much of his military activity was along the Baltic coast, in Pomerania. He defeated Count Dietrich of the Northern March at Cedynia in 972, and reached the mouth of the Odra river in 976. The decisive battle, fought in 979, ensured Mieszko's position as count of the march. The following year he celebrated his victory by dedicating the city of Gdansk at the mouth of the Vistula River, to compete with the ports of Szczecin and Wolin on the Odra.
In 981 Mieszko I lost the land known only as Grody Czerwienskie to Vladimir I, prince of Kiev. In 986 he pledged allegiance to the Emperor Otto III, and helped him with wars with the Polabians[?]. Shortly before his death he placed his state under the suzerainty of the Pope in a document usually called the Dagome Iudex.This Dagome Index indexes the lands of the former nun Oda, lists her husband as Dagome and her sons by him. In the seventeenth century a list of rulers over the Polish territories for the first time called them Piasts.
From his first marriage he had a son, his successor Boleslaus, and a daughter, Swiatoslawa, later the wife (as queen Sigrid the Proud) of Eric the Victorious[?], king of Sweden and then (as queen Gunhild) of king Sweyn Forkbeard of Denmark, and mother of king Canute of Denmark and England.
From his second marriage he had three sons, Mieszko, Lambert, and Swiatopolk.
see also History of Poland