John Haanen finished his MD at Leiden University cum laude in 1988. From 1988 till 1991 he performed his PhD studies at the Division of Immunohematology & Blood Bank of Leiden University Medical Center and at DNAX, Research Institute, Palo Alto, California, US and defended his PhD thesis on human CD4+ helper T1 cells reactive against mycobacteria in November 1991. He trained in internal medicine in hospitals in Leiden (university hospital) and The Hague (regional hospital).

In 1996 he was registered as internist. As staff member of the department of Internal Medicine at Leiden University Medical Center (LUMC) (Head: Prof Edo Meinders), he worked as internist at the internal medicine intensive care unit for the duration of one year (supervision by Prof Hans van der Hoeven).

From October 1997 - October 1999 he did a full-time post-doctoral fellowship at the Division of Immunology of the Netherlands Cancer Institute (NKI), Amsterdam, (Head: Prof Ada Kruisbeek) on preclinical tumour immunology and started his long-term collaboration with Prof Ton Schumacher.

From October 1999 – October 2001 he trained in medical oncology (Head: Prof Sjoerd Rodenhuis) and received a permanent position as (consultant) medical oncologist (division of medical oncology) and research group leader (division of immunology) at NKI.

His research has been focused on the development of innovative immunotherapies in close collaboration with Prof. Ton Schumacher. The Haanen lab developed a novel therapeutic DNA vaccination strategy from Bench to Bedside; The Haanen/Schumacher lab developed TCR gene therapy, again from Bench to Bedside. In 2008, he received an endowed professorship on Translational Immunotherapy of Cancer at LUMC.

From 2009 till 2018 he headed the Division of Medical Oncology at NKI. During this period, the division doubled in size, and clinical immunotherapy was introduced. In July 2018 he accepted a position as CSO Immunotherapy, one of the major research themes at NKI, to increase immunotherapy research, facilitate more translational research programs and start building the NKI Center for Cellular Therapy (solid cancers). His current research spreads over development of cellular therapies for solid tumours, neoadjuvant immunotherapies (renal cell cancer, involvement in GI cancers and head and neck cancers), and biomarker research. His clinical specialty is in melanoma and other skin cancers, kidney cancer and management of immune-related adverse events.

He has co-authored over 500 peer-reviewed articles, serves on the Editorial Board of ESMO Open, Cancer Treatment Reviews and Kidney Cancer. He is currently Editor-in-Chief of ESMO IOTECH. John Haanen was Scientific Co-Chair of ESMO Immuno-Oncology Congress from 2016-2019, and Scientific Chair of the ESMO 2020 Congress. He is or has been an active member of several ESMO Committees and Task Forces: Women for Oncology (W4O), Resilience Task Force, Gender Medicine Task Force, Integrated Immunotherapy Faculty Member and since 2019 serves as a member of the ESMO Council. Since 2017, he is a member of the Central Committee for Research involving Human Subjects (CCMO).