As head of the Leopoldian line, he ruled over the Inner Austrian duchies of Carinthia, Styria and Carniola as well as the County of Tyrol and Further Austria from 1396 until his death.

Born in Vienna, William was the oldest son of Duke Leopold III of Austria (1351–1386) and his consort Viridis Visconti (1352–1414), a daughter of Lord Bernabò of Milan.

This was one of the first attempts of the Habsburgs to extend their dominions by marrying heiresses, as Louis intended his elder daughter Mary and her fiancé Sigismund of Luxembourg to succeed in Poland, while Jadwiga was designated heir to the Kingdom of Hungary.

[2][3] They thought that the inexperienced William and his Austrian kinsmen could not safeguard Poland's interests against its powerful neighbours, especially the Luxemburgs which controlled Bohemia and Brandenburg, and had a strong claim on Hungary.

[6] Jogaila sent his envoys – including his brother, Skirgaila, and a German burgher from Riga, Hanul – to Kraków to request Jadwiga's hand in January 1385.

[13] Queen Elizabeth confirmed the previous agreements about the marriage, ordering Vladislaus II of Opole to make preparations for the ceremony.

[18][14] The official accounts of the municipal authorities of Kraków record that on 23 August 1385 an amnesty was granted to the prisoners in the city jail on the occasion of the celebration of the Queen's marriage.

The marriage did not produce any offspring and William did not live to see his wife succeed her brother Ladislaus of Naples as Queen Joanna II in 1414.