With the help of a Roman force sent from Gaul by Emperor Justinian, he defeated and killed King Agila near Seville in 534. Athanagild then became king of the Goths in Spain, and ceded a large portion of Spain to the Byzantines. He endeavoured to drive his Roman allies out of Spain, but was unsuccessful.
Although throughout his rule he had to fight the Byzantines, the Franks, and the Basques, Athanagild strengthened his kingdom internally by conciliating the Catholics, whom his Arian predecessors had oppressed. His court at Toledo was famed for its splendor. Athanagild was the father of the Frankish queens Brunhild and Galswintha. He was succeeded by his brothers Liuva I and Leovigild.