Brief biography paints St. Marcellus I as a rigorist who refused to accept the penitence of the lapsed, and says that Christian Rome rioted against him.

Long hagiography of Pope St. Marcellus I.

After a vacancy in office following the death of Pope St. Marcellinus, was elected to the papacy in 308. Fairly lengthy biographical article in the Catholic Encyclopedia.

Brief account of his pontificate, from "Popes Through the Ages" by Joseph Brusher.