The origins of the gens Neratia lie in the Italian town of Saepinum, located in Samnium. Olli Salomies, in his study of Imperial Roman nomenclature, has convincingly argued that while Marcellus was the brother of the jurist Lucius Neratius Priscus, and thus the natural child of Lucius Neratius Priscus, he had been adopted by his uncle Marcus Hirrius Fronto Neratius Pansa, consul in 73 or 74, who was childless.
An inscription recovered from Saepinum provides us the details of his career. Marcellus began in his teens as one of the tresviri monetalis, the most prestigious of the four boards that comprise the vigintiviri; assignment to this board was usually allocated to patricians or favored individuals. An inscription found in Xanthos is understood by experts to indicate that he accompanied honoring his adoptive father to the Roman province of Lycia while the older man governed as Legatus Augusti pro praetore (or imperial governor). Upon returning to Rome, both were adlected into the patrician class, most likely during the Roman census of 73/74; this promotion in status offered many opportunities, including being excused from holding some of the republican magistracies required to become consul.
Marcellus then was given the position of curator acta senatorum, or recorder of the minutes of the Senate; he is the first person attested to have held this position. Then at the age of 25, he held the republican magistrate of quaestor, being selected as one of the pair allocated to attend to the emperor; the duties of these quaestors included reading the Emperor's speeches to the Senate. This was followed by his admission to the collegia of the Salius Palatinus. Marcellus' service to the emperor resumed with a hitch as military tribune with the Legio XII Fulminata which was part of the expeditionary force led by his adoptive father Neratius Pansa in the region of Cappadocia. This expedition is thought to have taken place in 75 and 76.
Due to his promotion to the patrician class, Marcellus was excused from holding any commands or offices between the praetorship and his appointment as suffect consul in AD 95, replacing the emperor Domitian on the Ides of January. Ronald Syme describes being the suffect to replace the emperor as "high distinction, and close to being consul ordinarius." Marcellus served as curator aquarum urbis between his consulship and his appointment as the Legatus Augusti pro praetore of Roman Britain. He certainly held the office by AD 103, when his governorship is attested by a military diploma; he probably had succeeded Titus Avidius Quietus a year or two earlier.
About the time Marcellus was governor, the Roman army was hard pressed to hold the territory Agricola had conquered a generation before. Legio II Adiutrix had been withdrawn to the Danube in AD 92 with three cohorts of Batavian auxiliaries. Further, Emperor Trajan had need for reinforcements for his First Dacian War and had called for vexillations from the army of Britain. The natives took advantage of the understrength garrison: excavations at Newstead and elsewhere show signs of destruction by fire. Although the Romans often burnt what was not worth salvaging when they evacuated a fort, excavations also revealed human remains and immense quantities of equipment, including damaged armor, at Newstead showing clearly that it had been destroyed due to hostile action. All of this point to the frontier being withdrawn to the line of the Stanegate, a reorganization Marcellus certainly managed a porton.
Neratius Marcellus was a friend of Pliny the Younger; although none of Pliny's letters to Marcellus have survived, one of his existing letters does mention him. Pliny had requested that Marcellus make Suetonius a tribune in Britain; when he succeeded with obtaining this favor for Suetonius, the latter eventually declined the post, which Pliny then transferred to a relative of Suetonius in his place. This story indicates that Marcellus was able to make military appointments easily through the network of patronage and apparently without consulting the army.
Another illustration of Marcellus' role in the network of patronage of his time is a draft of a letter recovered from the Roman fort at Vindolanda. The commander of the Ninth Cohort of Batavians stationed there, Flavius Cerialis, writes to his friend Crispinus about meeting with governor Marcellus. The text is damaged at this point, and there have been several theories about the purpose of this communication. Anthony Birley suggested Cerialis was asking Crispinus to intercede on his behalf for a promotion or transfer; M. P. Speidel and R. Seider suggest this was an example of litterae commendaticiae, or a letter introducing himself to Crispinus; in their edition of the letter, Alan K. Bowman and J. David Thomas offer a more prosaic interpretation, "The writer is asking Crispinus to ... make his military service pleasant by putting him on good terms with as many influential people as possible."
Our next evidence for Neratius Marcellus comes decades after he returned from Britain, when he was elected consul ordinarius in AD 129, serving alongside Publius Juventius Celsus Titus Aufidius Hoenius Severianus. Observing that the emperor Hadrian relied on his brother for advice, Birley believes that Marcellus was "on familiar terms" with that emperor, which may be the reason for the signal honor of a second consulship. However, Birley then notes "it may be that he came to a sad end soon afterwards, for among Hadrian's close friends with whom he 'postea ut hostium loco habuit', the author of the Historia Augusta lists a Marcellus, forced to suicide by the emperor."
Neratius Marcellus was married twice: first to Corellia Hispulla, the daughter of Pliny’s elderly friend Quintus Corellius Rufus, suffect consul in AD 78; second to Domitia Vettilla the daughter of Lucius Domitius Apollinaris, suffect consul in 97. From his first marriage Marcellus had at least one son, Lucius Corellius Neratius Pansa, consul of AD 122; no children are known from his second marriage.