Dr Marshall is a general practitioner in Auckland and an Honorary Clinical Associate Professor in the Department of General Practice and Primary Care, Faculty of Health Sciences, University of Auckland. For a period in the 1970s he was a Police Surgeon and then Senior Police Surgeon, in Auckland. He is a Fellow of the Royal New Zealand College of General Practitioners, a Fellow of the New Zealand Medical Association and an Accredited Fellow of the Institute of Directors. He is a Past-President and Life Member of the Auckland Medico-Legal Society, Past-President of the New Zealand Medical Association and from 1995 till 2007, Chairman of ProCare Health Limited. He has appeared as an Expert Witness in various medical tribunals, having been called both by defence and prosecution counsel.