Robert Desmond Marshall was born in Nhill, Victoria, on 3 May 1926, one of five siblings who all became involved in medicine (Betty, a general practitioner, Gwen, a physiotherapist, and surgeons Vernon and Donald). After a brilliant student career at the University of Melbourne, he graduated in 1948 and trained at Royal Melbourne Hospital.

Bob lectured in anatomy at the University of Melbourne from 1951 until 1953, when he returned to Royal Melbourne Hospital. In 1955, he gained Fellowship of the Royal Australasian College of Surgeons (RACS) and received his Master of Surgery from the University of Melbourne.