He was a Greek, and his enrolment among the protectores domestici (household guards) shows that he was of noble birth. He entered the army at an early age, when Constantius II was emperor of the East, and was sent to serve under Ursicinus, governor of Nisibis and magister militiae.
He returned to Italy with Ursicinus, when he was recalled by Constantius, and accompanied him on the expedition against Silvanus the Frank, who had been forced by the unjust accusations of his enemies into proclaiming himself emperor in Gaul. With Ursicinus he went twice to the East, and barely escaped with his life from Amida or Amid (modern Diarbekr), when it was taken by the Persian king Shapur II. When Ursicinus lost his office and the favour of Constantius, Ammianus seems to have shared his downfall; but under Julian the Apostate, Constantius's successor, he regained his position. He accompanied this emperor, for whom he expresses enthusiastic admiration, in his campaigns against the Alamanni and the Persians; after his death he took part in the retreat of Jovian as far as Antioch, where he was residing when the conspiracy of Theodorus (371) was discovered and cruelly put down.
Eventually he settled in Rome, where, at an advanced age, he wrote (in Latin) a history of the Roman empire from the accession of Nerva to the death of Valens (A.D. 96-378), thus forming a continuation of the work of Tacitus. This history (Rerum Gestarum Libri XXXI) was originally in thirty-one books, but the first thirteen are lost. The surviving eighteen books cover the period from 353 to 378. As a whole it is extremely valuable, being a clear, comprehensive and impartial account of events by a contemporary of soldierly honesty, independent judgment and wide reading. Edward Gibbon judged Ammianus as "an accurate and faithful guide, who composed the history of his own times without indulging the prejudices and passions which usually affect the mind of a contemporary." Although Ammianus was no doubt a pagan inclining to neo-Platonism, he shows no prejudices against Christianity. His style is generally harsh, often pompous and extremely obscure, occasionally even journalistic in tone, but the author's foreign origin and his military life and training partially explain this.
Further, the work being intended for public recitation, some rhetorical embellishment was necessary, even at the cost of simplicity. It is a striking fact that Ammianus, though a professional soldier, gives excellent pictures of social and economic problems, and in his attitude to the non-Roman peoples of the empire he is far more broad-minded than writers like Livy and Tacitus; his digressions on the various countries he had visited are peculiarly interesting.
In his description of the empire--the exhaustion produced by excessive taxation, the financial ruin of the middle classes, the progressive decline in the morale of the army--we find the explanation of its fall before the Goths twenty years after his death.