Herbord Osl grew up in the royal court of Andrew II together with Duke Béla, therefore possibly he was born in the early 1200s. He belonged to the group of so-called "royal youth" (, ), who supported the monarchs and took a leading role in royal military campaigns. He was first chronologically mentioned by contemporary documents in 1230, when participated in the military campaign of the duke, who crossed the Carpathian Mountains and laid siege to Halych together with his Cuman allies. According to King Béla's charter from 1248, he "fought appreciatively" at the "Porta Rusica" (lit. "Russian Gate"), then participated in the skirmish along the river Dniester. It is possible that he also fought in the post-1235 wars against the Kingdom of Galicia–Volhynia, following Béla IV asenceded the Hungarian throne after his father's death. During the First Mongol invasion of Hungary, when Béla escaped after the disastrous Battle of Mohi, took place on 11 April 1241, and fled to the coast of the Adriatic Sea, Herbord left behind his estates and relatives and joined to the royal companion to Croatia and Dalmatia by the end of June. The king took refugee in the well-fortified Trogir and entrusted Herbord to protect his eldest son Stephen at the fortress of Klis. Subsequently, he was one of the nobles, who protected the western boundary against the raids of Frederick the Quarrelsome. Herbord besieged and successfully recaptured the castle of Kőszeg with his own army from the Austrian and Styrian troops, according to the aforementioned royal charter of 1248. For his success and loyalty, he was granted landholdings beyond the Drava river.