Upon graduation in 1983, Tan started his medical career as a renal physician. From 1987, he took on concurrent hospital and academic roles beginning as a faculty member with the Department of Medicine at NUS and Registrar, Division of Nephrology, Department of Medicine at
National University Hospital (NUH). In 1997, he became a Senior Consultant with the Division of Nephrology at NUH, and in 1999, a Professor of Medicine with NUS. He was the Chief and Head of the Department of Medicine at both NUS and NUH from 1997 to 1998. Subsequently, he became the Dean of Faculty of Medicine, NUS and also Chairman of the NUH Medical Board from 1997 to 2000. Tan was appointed as the Director of Medical Services at MOH from 2000 to 2004. During his tenure, he led the public health response to SARS in 2003. Tan served as the NUS's Provost and Deputy President between 2004 and 2007 and then as Senior Deputy President before assuming the appointment of President from December 2008 to December 2017. Tan was also responsible for setting up, in partnership with
Yale University, the
Yale-NUS College, Singapore's first liberal arts college. In 2008, Tan served as the chief executive for the
National University Health System, bringing the various academic health institutions such as
Yong Loo Lin School of Medicine, Faculty of Dentistry, and health institutions such as
National University Hospital under an unified governance. Tan also served on Singapore's
Agency for Science, Technology and Research's (A*STAR) board as a deputy chairman from 2004 to 2017. , he was also the chairman of A*STAR's Committee of Government Scientific Advisors. Since January 2018, Tan has been the Chief Health Scientist at MOH and Executive Director of its new Office for Healthcare Transformation, formed in 2018 to help accelerate, drive and contribute to the transformation of Singapore's healthcare system. ==Other activities==