Before Sigismund I ruled as king of Poland, he had already been invested as duke of Silesia.
The son of king Casimir IV of Poland and Elizabeth Habsburg of Austria, Sigismund followed his brothers John Albert[?] and Alexander[?] on the Polish throne. Their eldest brother Louis II of Bohemia ruled as Ladislaus (Ulaszlo) II[?] of Hungary and Bohemia.
Sigismund faced the challenge of consolidating internal power to face external threats to the country. During Alexander's rule, a law 'Nihil Novi' had been instated, that forbade Kings of Poland from enacting laws without the consent of the 'diet'. This proved crippling in Sigismund's dealings with his nobles and magnates.
Despite this Achilles heel, he established (1527) a conscription army, and the burocracy necessary to finance it.
Intermittantly at war with Vasily III of Moscow, starting in 1507 (before his army was fully under his command), 1514 marked the fall of Smolensk (under Polish domination) to the Muscovite forces (which lent force to his arguments for the necessity of a standing army). 1515 he entered an alliance with the Holy Roman Emperor Maximilian I.
In return for Maximilian lending weight to the provisions of the 2nd Peace of Toru, Sigismund consented to the marriage of the children of Uladislaus II of Bohemia and Hungary, his brother, to the grandchildren of Maximilian. Through this double marriage contract Bohemia and Hungary passed to the House of Hapsburg in 1526, on the death of Sigismund's nephew, Louis II.
The Polish wars against the Teutonic Knights ended in 1525, when Albert of Brandenburg, their marshal, converted to Lutheranism, secularized the order, and paid homage to Sigismund, who in return gave him the domains of that order, as the First Duke of Prussia. A daughter of Sigismund I, Hedwig (1513-1573), married Joachim II, Margrave of Brandenburg[?].
In other politics, Sigismund sought a peaceful coexistence with the Khans of Crimea, but was unable to completely end border skirmishes.
Sigismund was a Humanist (c.f. David Hume). He and his 2nd wife, Bona Sforza, daughter of Gian Galeazzo Sforza of Milan, were both patrons of Renaissance culture, which began, under them, to flourish in Poland
On Sigismund's death, his son Sigismund Augustus became the last Jagiellonian king of Poland.