Marcellus is a town in Onondaga County, New York, United States. The population was 6,210 at the 2010 census. The town was probably named after Marcus Claudius Marcellus, a Roman general, by a clerk interested in the Classics.The Marcellus Formation is a vast geological layer of shale spanning Pennsylvania, West Virginia and parts of other states and Ontario, which is named for an outcropping in or near Marcellus.The Town of Marcellus contains a village also named Marcellus. The town and village are southwest of Syracuse.