We draw our notices about this panegyrist, born at the start of the 4th c. (he is presented, in fact, as a senior man), from the same Gratiarum actio that was given to thank Julian for the consulate that was granted to him and from Ammianus Marcellinus, who appointed him several times in connection to the events that occurred in 361 (Rerum gestarum libri 21.10.8; 21.12.25). There is no proof that he was in any way related to the panegyrist Mamertinus from the preceding century. Claudius Mamertinus was of Gallic origin, or at least had lived in Gaul for a long time; his preference for Latin, itself unusual for a follower of an emperor who favored Greek, also confirms that he came from the west (Nixon-Rodgers). He probably served in the imperial administration of Gaul, but the first notices arise in 361, when he was appointed by Julian to be comes sacrarum largitionum and praetorian prefect in Illyricum. He followed the emperor in the East when these men rebelled against Constantius II and, after the latter’s death, also obtained the prefectures of Africa and Italy, becoming consul for 362 together with the general Flavius Nevitta, and in this way obtaining senatorial rank. After the death of Julian, he also remained in charge under Jovian, Valentinian I, and Valens; he received the task of overseeing the restoration to the imperial budget of the assets that Julian had allocated to the pagan temples. After being accused of embezzlement in 365, he lost his post (Amm. 27.7.1). We have no later notices about him, nor do we have information about his religion, though his defense of philosophy and his relationship with Julian make paganism likely. Endowed with good rhetorical training, he was familiar with Greek authors and showed a special predilection for Cicero. [A. Balbo; tr. C. L. Caterine].