A rival of Fritigern, another Therving war chief, Athanaric made his first appearance in recorded history in 369, when he engaged in battle with the Eastern Roman Emperor Valens and ultimately negotiated a favorable peace for his people.
During his reign, the Thervings were divided by religious issues. Many of them had converted to Arian Christianity during the third and fourth centuries, but Athanaric continued to follow the old Germanic pagan religion. Fritigern, his rival, was an Arian and had the favor of Valens, who shared his religious beliefs.
According to Socrates, Fritigern and Athanaric were rival leaders of the (Therving) Goths. As this rivalry grew into warfare, Athanaric gained the advantage, and Fritigern asked for Roman aid. The Emperor Valens and the Thracian field army intervened, Valens and Fritigern defeated Athanaric, and Fritigern converted to Christianity, following the same teachings as Valens followed. Sozomen follows Socrates' account.
According to Zosimus, Athanaric (Athomaricus) was the king of the Goths (Scythians). Sometime after their victory at Adrianople, and after the accession of Theodosius, Fritigern, Alatheus, and Saphrax moved north of the Danube and defeated Athanaric, before returning south of the Danube.
In 376, Valens permitted Fritigern's people to cross the Danube River and settle on Roman soil to avoid the Huns, who had recently conquered the Greuthungs and were now pressing the Thervings then living in Dacia. Athanaric's people were left to their fate, but many of them found their own way across the river, as well.
By 379, one year after Fritigern's great victory over the Romans at the Battle of Adrianople in 378, he had won over most of the Thervings to his leadership. But he died a year later, and Athanaric became king of the entire Therving people.
Shortly before his death in 381, he became the first foreign king to visit the new Roman capital of Constantinople. He negotiated a peace with the new emperor, Theodosius I, that made some Thervings foederati, or official allies of Rome allowed to settle on Roman soil as a state within a state.
A few weeks later, Athanaric died, but the treaty he had brokered stood until Theodosius' death in 395.