His accession marked the start of the Jagiellonian dynasty, which was to rule both countries until 1572, presiding over their administrative union in 1569. The Jagiellonian era is usually characterised as the start of Poland's "golden age", when the country became a major European power and extended its frontiers to the north and east.
Jagiello's conversion marked the establishment of Christianity as the official religion of Lithuania following a brief period of Catholic kingship in the 1250s. Though pagan beliefs continued in the country for some time, the Lithuanian nobility gave its support to the new order upon being promised (in 1401) a voice in the election of the future Polish-Lithuanian rulers.
Jogailla or Jagiello is reported to have married Hedwig / Jadwiga, queen of Poland. He first co-ruled with her since 1386. After Queen Jadwiga's death in 1399 he was ruler until his death in 1434 under the name Władysław II Jagiełło (Ladislaus II).
As Ladislaus II, his reign is noted for the crushing defeat inflicted on the Teutonic Knights in neighbouring Province of Prussia by Polish, Lithuanian, Russian and Tatar forces at the Battle of Grunwald 1410.
Jogaila was the first of the dynasty of Lithuanian dukes who were also kings of Poland (1386-1572). He was succeeded by his son Ladislaus III, and after his death at battle of Varna by second son Casimir IV.
Before that the Piasts and the Przemysls were intermittent kings of Poland.
Although Jagiellonian dynasty was not hereditary and in theory each member of dynasty had to be elected, in reality every time when the father died, his son was elected as new king.
After the last Jagiellonian dynasty member died out, the kings of Poland were elected by the Polish nobles.