Mieszko I (c. 935-May 25 992), son of the semi-legendary Siemomysl, was the first (historically known) Piast duke of the Polans, which gave that name to a country later called Poland. Mieszko was not his actual name, but given at a later time.

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 according to the government system of that time, to receive the office as duke by the emperor(s). 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). 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.

Mieszko I had pledged allegiance to imperial margrave Gero ,who died in 965, then directly to emperor Otto I the Great, to emperor Otto II and again to emperor Otto III, however there is much dispute from the Polish side over this fact - mainly whether he was vassal from whole Poland, or from part Poland (the disputed fragment is "usque Varta fluvium"). Some question by Poles is whether Mieszko pledged allegiance to margrave Gero, since there is only one mentioning in one chronicle about that fact, moreover which is abstract of earlier chronicle where there is nothing about it.

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 territory later called Pomerania. He defeated Count Dietrich of the Northern March at Cedynia in 972, and reached the mouth of the Oder/Odra river in 976. The decisive battle, fought in 979, ensured Mieszko's position as count of the march, a position given by the empire. The following year he celebrated his temporary conquest by dedicating a fortress at Gdansk. Settlements there have existed for millenniums and Pomeranian and Prussian territories overlap at the mouth of the Vistula River. Mieszko's attempt was conquest of the Baltic coast as far as he could get away with. Mieszko attempted to conquer the Prussians for decades, but after one of his attempt by sending Adalbert of Prague had failed in 997, the settlement in the Adalbert histories was recorded as Gedanum.

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 Iudex indexes the lands of the Mieszko, refered as "Dagome" in document, and his wife, former nun Oda and her sons by him.

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.