In 544, Marcellus was appointed a general, along with Constantinianus, in the ongoing Lazic War (541–562) against Sassanid Persia, replacing the killed generals Justus (a cousin of Justinian and thus Marcellus's uncle) and Peranius. According to Procopius, at this time Marcellus was still very young, "just arriving at the age of manhood". No activities of his are recorded there, however, and the war soon ended by a truce.