Marwick was born near Oamaru, New Zealand, on 3 February 1891, the son of Hugh Marwick, and his wife, Jane née Cuthbert. While at Waitaki Boys' High School he helped to collect fossil shells and learned the beginnings of how to classify molluscs. He studied and taught at the University of Otago, and in 1912 gained an MA with first-class honours in with a thesis on geology.