Lewis was an instructor of Radiology at the Washington University School of Medicine, Mallinckrodt Institute of Radiology from 2000 to 2002. From 2003 to 2008, he was an assistant professor of radiology at the Washington University School of Medicine, Mallinckrodt Institute of Radiology. He is the Emily Tow Jackson Chair in Oncology, the Vice Chairman for Research (Radiology) and Chief Attending of the Radiochemistry and Imaging Sciences Service at Memorial Sloan Kettering Cancer Center. He also heads a laboratory in the Sloan Kettering Institute's Molecular Pharmacology Program and is a professor at the Gerstner Sloan Kettering Graduate School of Biomedical Sciences.
Lewis holds joint appointments in the Departments of Radiology and the Department of Pharmacology at Weill Cornell Medical School, NY.