Ammianus Marcellinus underlines the fact that Marcellus was a relative of Procopius (a member of the Constantinian dynasty).
In 365 Procopius rebelled against emperor Valens, taking the purple; Marcellus became his protector and was entrusted with the defence of Nicaea.
Valens' magister militum, Equitius, learned of Procopius' death and attacked Marcellus, capturing him.
According to Zosimus, Marcellus was sent by Procopius from Constantinople into Bithynia, to counter the cavalry unit under Serenianus' command and loyal to Valens.
According to Zosimus and John of Antioch, Marcellus was suspected of planning a usurpation, as he had been found with the regalia of Procopius; thus Valens ordered his death and that of his family and his supporters.