Radonja was a member of the Branković dynasty as the eldest son of Branko Mladenović. Radonja's younger brothers were Vuk Branković and Grgur Branković. He was married to Jelena, a sister of Uglješa Mrnjavčević. Radonja had a title of caesar (Serbian: ћесар) and controlled an estate in Serres region where he and his wife Jelena lived with their two daughters.
When his wife and daughters died at very young ages, Radonja resigned his feudal position and after Autumn 1364 took monastic vows and name Gerasim while his father Branko Mladenović was still alive.There he served and helped with great merit Hilendar and also the monasteries of Koutloumousiou and Agiou Pavlou monastery. Thanks to the Radonja's influence, Vuk Branković became the first of Serbian feudal lords who gained ktitor's privileges in Hilandar after the fall of the Serbian Empire, beginning in 1371. The earliest mention of Radonja under his monastic name Gerasim was in a charter of Vuk Branković issued to Hilandar Monastery in 1376/77. Around 1380 Radonja, together with Arsenije Bagaš, bought ruined Agiou Pavlou monastery from Xeropotamou Monastery and reconstructed it. In period 1379—89 Radonja was a priest in the Hilandar Monastery and was again mentioned as Gerasim, a brother of Vuk Branković, in the 1389 document written by Prince Lazar. There he achieved hieroschemamonk degree of Eastern Orthodox monasticism. Around 1392 Radonja and group of notable "elders" from Hilandar visited court of the Lazarević family, successors of Prince Lazar, and requested a church in Ibar and its estates to be granted to Hilandar, according to the promise given by its former lord, Obrad Dragoslavić.
After the death of Vuk Branković in 1396, Radonja took Vuk's body to the Agiou Pavlou monastery on Mount Athos.