Nothing of his life before the papacy has survived the centuries. Pope at the end of the persecutions of Diocletian; the persecutions had so disrupted the Church that there had been a gap of over a year with no Pope. Faced reconsituting the clergy which had been decimated and whose remnant had practiced their vocation covertly and with the expectation of martyrdom. Worked to recover and welcome back those who had denied the faith to keep from being murdered.
When a group of the apostacized, known as the Lapsi, refused to do penance, Marcellus refused to allow their return to the Church. This group had some political pull, and some caused such civil disruption that Emperor Maxentius exiled the Pope in order to settle the matter. Legend says that Marcellus was forced to work as a stable slave as punishment, but this appears to be fiction. Considered a martyr as he died of the terrible conditions he suffered in exile.
309; initially buried in the cemetery of Saint Priscilla in Rome; relics later translated to beneath the altar of San Marcello al Corso church in Rome where they remain today