Mae Marcus yn Athro Emeritws ym maes Polisi Iechyd ym Mhrifysgol De Cymru, ac yn Ddirprwy Gadeirydd yr Awdurdod Safonau Proffesiynol ar gyfer Iechyd a Gofal Cymdeithasol. Bu'n Gadeirydd Bwrdd Iechyd Prifysgol Cwm Taf Morgannwg rhwng 2017 a 2021, ac am bedair blynedd cyn hynny bu'n Is-gadeirydd Bwrdd Iechyd Caerdydd a'r Fro.  Cafodd ei addysg ym Mhrifysgolion Rhydychen, Caerdydd a Bryste, ac fe'i hetholwyd yn Gymrawd Cyfadran Iechyd Cyhoeddus Colegau Brenhinol y Meddygon yn 2008.
Marcus is Professor Emeritus in Health Policy at the University of South Wales, and Deputy Chair of the Professional Standards Authority for Health and Social Care.  He was Chair of Cwm Taf Morgannwg University Health Board from 2017 to 2021, and for four years before that Vice Chair of Cardiff and the Vale Health Board.  He was educated at the Universities of Oxford, Cardiff and Bristol, and was elected a Fellow of the Faculty of Public Health of the Royal Colleges of Physicians in 2008.