Gratian was born at Sirmium in AD 359, the son of Valentinian and Marina Severa. He became consul in 366, co-augustus in 367 and sole emperor on 17 November AD 375 at his fathers death. His lone reign lasted for a mere five days, after which his brother Valentinian II was hailed co-augustus at Aquincum because of inter-legionary jealousies.
Soon leading court figures as Theodosius the Elder and Maximus were put to death for treason. After this government came to rest with Ausonius, originally a poet, who created a policy of broad religious toleration. Ausonius even reconsulted the senate.
While campaiging against the Alemanni in 377-378 Gratian heard of Valens' troubles with the Visigoths. He was however too late to be of aid so at Hadrainople Valens and his army were totally wiped out.
In these years Ambrose, bishop of Mediolanum (Milan) became more and more powerful and started a policy of extreme persecution of all (non) christian heresies. During this period Gratian dropped the title of Gratian pontifex maximus. Because of this religious intolerance Gratian's popularity fell drastically.
In AD 383 news reached Gratian that Magnus Maximus had been hailed emperor in Britain and had crossed the Channel into Gaul. Gratian tried to stop him but was deserted by his soldiers. The fleeing emperor was assassinated by a close friend of Maximus (Andragathius) while posing as a true friend of the refugee.(August AD 383).