Marshall was born in Kalgoorlie, Western Australia and lived in Kalgoorlie and Carnarvon until moving to Perth at the age of eight. His father held various jobs, and his mother was a nurse. He is the eldest of four siblings. He attended Newman College and the University of Western Australia, where he received a Bachelor of Medicine, Bachelor of Surgery (MBBS) in 1974. He married his wife Adrienne in 1972 and has four children