He was "a former soldier and a Greek" ut miles quondam et graecus (Amm. 31.16.9) he tells us, and his enrollment among the elite 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 in Mesopotamia, and magister militum.
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 allegedly 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 (modern Diyarbakır), when it was taken by the Sassanid king Shapur II. When Ursicinus lost his office and the favour of Constantius, Ammianus seems to have shared his downfall; but under Julian, 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 Sassanids; after the death of Julian, 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.
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" (Gibbon 26.5). Ammianus was a pagan, and some have said that he marginalises Christianity repeatedly in his account. On the other hand, Edward Gibbon condemned Ammianus for lack of literary flair and found his dedication to accuracy, "tedious and disgusting. Ernst Stein, however, goes as far as praising Ammianus as "the greatest literary genius that the world produced between Tacitus and Dante".
Scholars often traditionally believed that Ammianus' work was intended for public recitation for two reasons: the overwhelming presence of a accentual clasulae (cursus), whose presence could supposedly only have been detected with pronunciation; and the epistle 1063 of Libanius to a Marcellinus of Rome which refers to public recitations. However, recent studies of Greek works have shown that virtually all of them possessed such a pattern, so its presence means nothing as far as the intended audience of Ammianus' work. Few scholars accept that the letter of Libanius was addressed to Ammianus, since Marcellinus was a very commmon name and the tone suggests Libanius was addressing a man much younger than himself (not a man such as Ammianus, who would have been his contemporary). 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 particularly interesting.
Ammianus' work contains a detailed description of the 365 A.D. Alexandria tsunami which devastated the metropolis and the shores of the eastern Mediterranean on 21 July of that year. His report describes accurately the characteristic sequence of earthquake, retreat of the sea and sudden giant wave.
His work, the Res Gestae, has suffered terribly from the manuscript transmission. Aside from the loss of the first thirteen books, the remaining eighteen are in many places corrupt and lacunose. The sole surviving manuscript from which almost every other is derived is a ninth-century Carolingian text, V, produced in Fulda from an insular exemplar. The only independent textual source for Ammianus lies in M, another ninth-century Frankish codex which was, unfortunately, unbound and placed in other codices during the fifteenth century. Only six leaves of M survive; however, the printed edition of Gelenius (G) is considered to be based on M, making it an important witness to the textual tradition of the Res Gestae. See Clark, Text Tradition.