Radu was the only son of Nicholas Alexander of Wallachia by his second wife, Clara Dobokai. He was co-ruler with his half-brother Vladislav I at least from 1372. He may have started his reign as sole ruler after 9 July 1374 when the last documentary mention of his brother as being alive is dated. Unfortunately, there aren't any internal documents that could attest his reign. There are a few external mentions of him, primarily documents of the Kingdom of Hungary and a contemporary Italian chronicle and also a late pisanie[clarification needed], an inscription on the walls of Curtea de Argeș Cathedral and an important quantity of coins (ducati, dinari and bani) which were emitted by the voievod .
The relations with the Hungarian kingship during his reign remained tense, resulting in armed conflicts. The details of these fights remain unclear. An Italian chronicle "Cronaca Carrarese" tells about an expedition of the Hungarian king Louis the Great between 5 July - 14 August 1377 with the intention of subduing a one "Radano principe di Bulgaria infedele" (Italian), identified with Radu I. These tense relations which preceded the fight are confirmed by the registers of the Venetian Republic where, in the same year, a large order of full-equip armors made by the Wallachian voievod appears. These armors were to be worn by those "armadura da cavali", 10.000 at number who were defeated in battle against the Hungarian king as pointed out by the Italian chronicle.
It is hard to tell whether the troops of the Wallachian voievod were truly defeated, as from the internal documents of the Kingdom of Hungary which were published in the next period, it appears that the Hungarian king still had the intention of subduing the unfaithful voievod. Thus on 19 November of the same year, the king promised the Saxon traders of Transylvania from Brasov that if Wallachia becomes his possession he reduces the dues. On the contrary, in an order given to the landlord of Orsova in 1382 through which he was obliged to forbid any foreign trader to enter Wallachia with merchandise and to keep guard of the border "day and night" reporting everything that happened, there is no mention of a Hungarian ban of Severin and Orsova appears as border point thus suggesting that the Eastern part of Banat was annexed by the Wallachian voievod Radu I.
Maybe linked to this fact is the strange name given to the voievod in the biography of Sigismund of Luxembourg in which, speaking of Vlad Dracul, it is said that he was the son of "Merzeweydan" (Mircea) and grandson of "Pankraz dem Weisen" (Pankraz the Wise). The name Pankraz (Pancratius) is explained from the deterioration of the name Radu Ban (ban of Severin, title taken after the conquest of the territory). Nevertheless, the Wallachian voievod, in another Hungarian document which is contemporary with the events of 1377, bears the name "Godon" as well strange and unclear.