Even people who know next-to-nothing about the history of the later Roman Empire, usually know the names of Constantine the Great and Julian the Apostate, the one because he set Christianity in place in the Roman Empire, and the other, because he tried to reverse the trend toward Christianity. Another reason we know about Julian is that contemporary writers whose works have survived, wrote about him.

Among these writers is the historian Ammianus Marcellinus, who -- G.W. Bowersock claims in his biography of Julian the Apostate -- in addition to relying on first-hand experiences, may have used the now lost biographical material of Eunapius of Sardis. Eunapius, who was privy to the memoir of Julian's friend and physician, Oribasius, was also the principal source for the later historian, Zosimus.
What we know of Ammianus Marcellinus comes mostly from his own Latin writing, but the details are limited. We do not know, for instance, when or where he was born or when or where he died.

Ammianus Marcellinus, the author of a Res Gestae in 31 books, says he is a Greek. David Rohrbacher, in his The Historians of Late Antiquity, says that being a native of the Syrian city of Antioch (an assumption Bowersock questions) predisposed Ammianus to write in Latin. Rohrbacher says that the fact that Ammianus achieved the position of protector domesticus under Ursicinus, as a young man (before 350), means that his family had connections, which would go along with the familiarity Ammianus had with Latin, despite being Greek.

Ursicinus was magister equitum in the East, but was sent to Gaul. Ammianus accompanied him. He says Emperor Constantius II asked the two of them to befriend a usurper in Gaul (Silvanus) and then assassinate him. After they succeeded in their mission, Ursicinus became general in Gaul. Since Ammianus was with him, this provided the opportunity to witness Julian the Apostate at first hand, since it was in Gaul that Julian achieved his early military successes. In 356, Marcellus replaced Ursicinus, but Ursicinus and Ammianus remained in Gaul until the emperor summoned them to Sirmium, in northern Italy, in 357.

Ursicinus became magister equitum in the East again, but was then recalled to Italy in 359. Before Ursicinus and Ammianus had reached Italy, they were told to return to the eastern front to fight the Persians. During the conflict, the Persians besieged the city of Amida, where Ammianus was stationed. The siege lasted 73 days before the city fell, but the Persians were tired out by it, as well. Ammianus was able to escape to Melitina, in Armenia, where he met a general who helped him back to Antioch.

The fall of Amida led to Ursicinus' retirement. What happened to Ammianus at that point is unknown, although Rohrbacher quotes Ammianus' contemporary Libanius to say that Ammianus continued to dress as a soldier but act like a Greek philosopher. After Julian's revolt, it is likely that Ammianus joined his troops when the emperor arrived in Antioch in 363. Ammianus was present when Julian the Apostate fatally invaded Persia.

Following the death of Julian, and his successor Jovian's retreat, Ammianus continued his history for about another 20 years.

Sources:
The Historians of Late Antiquity, by David Rohrbacher
Julian the Apostate, by G.W. Bowersock