John was a high ranking civil servant of Gothic origin at the court of emperor Honorius.
As Honorius had made his incestuous advances toward his half-sister Galla Placidia, the scandal and crisis brought about by this at court saw Castinus, the 'Master of Soldiers' of the western empire, backing Honorius and hence making an enemy of Galla Placidia.
Galla Placidia fled her half-brother to the safety of Constantinople.
Therefore when Honorius died, Castinus quite naturally did not want to see the return of Galla Placidia, for it would have no doubt meant the end of his own power. Placidia of course was the mother of Valentinian III, son of Constantius III, the obvious heir to the western throne. Her return would have made the four year-old Valentinian III emperor and his mother Placidia the regent on his behalf.
In order to prevent Placidia's return, Castinus in September AD 423 simply proclaimed John emperor of the west.
Though Theodosius II, emperor of the east, refused to accept this arrangement, despite strenuous attempts being made to gain his recognition.
To his credit, John appears to have been a moderate and mild ruler.
Alas, Theodosius dispatched an expedition to rid the empire of the usurper and establish Valentinian III on the western throne.
The fleet which set sail was under the command of the emperor's best general of the day, Ardaburius. But the fleet was hit by a storm and Ardaburius himself was shipwrecked and taken prisoner as he was washed ashore near Ravenna.
Ardaburius' son Aspar however landed the remaining forces and conquered Aquileia.
John now failed to act immediately. He was waiting for the arrival of his general Aetius with an army of Huns.
Meanwhile Ardaburius used his apparently very unrestrained captivity in Ravenna to undermine the loyalty of John's officers. He also managed to send message to his son Aspar, in which he urged him and his forces to advance on Ravenna.
Aspar advanced carefully to remain undetected as he slowly moved his troops towards Ravenna. Secretely they were guided through the marshes around Ravenna by a shepherd, closing on the city they remained unnoticed by its defenders. Supporters within the city, most likely prompted by Ardaburius, opened the gates and the army flooded into Ravenna unopposed.
John was arrested and, at Aquileia, was condemned to death by Galla Placidia.
His right hand was cut off and he was paraded around the circus, tied onto a donkey, for ridicule by the public.