Marc Ansari studied medicine in Geneva, after a degree in biology in 1996. He obtained his medical diploma in 2001, which he complemented with a specialisation in paediatrics. He then moved to Canada, where he pursued his clinical training in paediatric oncology and haematology, with a particular focus on stem cell transplantation. He also conducted research in pharmacogenomics at the Cancer Research Centre of CHU Sainte-Justine. Upon returning to Geneva, he founded the Paediatric Palliative Care Group (GPS). In 2008, he obtained a Swiss specialisation in paediatric onco-haematology and joined the Paediatric Onco-Haematology Unit at HUG, which he has led since 2013.

His research interests focus primarily on pharmacogenomics and personalised therapy in paediatric onco-haematology. In 2011, he founded the Paediatric Haematology and Oncology Research Platform at the Faculty of Medicine, as well as the CANSEARCH Foundation to support its activities. The platform specialises in research on neuroblastoma, central nervous system tumours, liver tumours, stem cell transplantation, long-term outcomes for childhood cancer survivors, and various other childhood tumours. Since 2014, he has served on the European board of the paediatric working group of the Blood and Marrow Transplantation society (EBMT PWD) and of the European Society for Pharmacogenomics and Individualised Therapy (ESPT). In 2015, he established the first paediatric germline DNA biobank at HUG (BaHOP), and in 2016, the Swiss Group for Pharmacogenomics and Individualised Therapy. In 2018, he was appointed Chair of the EBMT PWD Clinical Trial Scientific Committee. Appointed Assistant Professor in the Department of Paediatrics,Gynaecology and Obstetrics at the Faculty of Medicine in 2016, he was promoted to Associate Professor in 2018 and to Full Professor in May 2026.