Ladislaus was born into the Dezsőfi branch of the influential Losonci family as the son of Desiderius I, castellan of Kőszeg then Sebesvár (today Bologa in Poieni, Romania).

[4] In this capacity, according to a royal charter issued by Louis I in March 1373, Losonci committed "official misconduct" when prevented the Saxons of Kronstadt (present-day Brașov, Romania) to use their forests within the border of their privileged seat in Burzenland.

Both the present-day Romanian and Hungarian names of the village, Voivodeni and Vajdaszentivány, respectively, preserved his central role in the development and history of the settlement.

Alongside Lord Chancellor Demetrius and Ban Nicholas Szécsi, Losonci took part in defining the boundaries of estates across the province, initiated by Louis.

However they remained unpopular among the Hungarian noblemen, the majority of whom regarded Mary's distant cousin, Charles III of Naples, as the lawful king.

[3] Returning home through Rome, he obtained permission from Pope Urban VI to found an Augustinian monastery near the Parish Church of Szászrégen (today Reghin, Romania).

In addition, Mary renounced the crown without resistance in December in favour of Charles III of Naples who was elected King of Hungary by the Diet.

[6] He participated in the siege of Novigrad Castle, where Sigismund's troops captured the fort and liberated Mary on 4 June 1387 (her mother Elizabeth was strangled in January).

[12] Sigismund donated Csicsó Castle (Romanian: Cetatea Ciceu) to Ladislaus and Nicholas Losonci for their merits in the struggle against Charles' supporters.

[13] He was also granted Bálványos Castle (today ruins in Unguraș, Romania) around 1387, which remained his family's property until the death of his son John.

Voivode Losonci and his kinship launched a military campaign from Transylvania to Buda in August 1390, destroying their opponents' possessions on the way.