On the death of the Emperor Honorius (August 27, 423), Theodosius II, the remaining ruler of the House of Theodosius hesitated for some time in announcing his uncle's death and in the interregum Honorius' patrician at the time of his death, Castinus, elevated Joannes as emperor.
Joannes was a primicerius notariorum or senior civil servant at the time of his elevation. Procopius praises his mildness, intelligence, and general ability. Unlike the Theodosian emperors, he tolerated all Christian sects. His control over Gaul was insecure: his pretorian prefect of that region was slain at Arles in an uprising of the soldiery there. And Count Boniface, in control of the African provinces, held back the grain fleet from Rome.
Joannes had hoped that he could come to an agreement with the emperor Theodosius, but when Theodosius elevated the young Valentinian III first to Caesar, then to co-emperor as an Augustus (undoubtedly influenced by Valentinian's mother Galla Placidia), he knew he could only expect war. Late in 424, he sent one of his younger, but promising, followers Aetius on an embassy to the Huns to seek military help.
While Aetius was away on his mission, the army of the Eastern Empire left Thessalonica for Italy, and soon made their base in Aquileia. Further military actions were inconclusive until the garrison of Ravenna was convinced to betray him to the Imperialists. The fallen emperor was brought to Aquileia where first his hand was cut off, then he was paraded on an ass in the Hippodrome to the insults of the populace, then after further insults and injuries he was decapitated in June or July of 425.
Three days after Joannes' death, Aetius returned at the head of a substantial Hunnic army. After some preliminary skirmishing, Placidia and Aetius came to an agreement: the Huns would be paid off and sent home, while Aetius would receive a senior rank in the Roman army. Thus the political landscape of the Western Roman Empire for the next thirty years was established.