Roy's career at Tata Institute of Fundamental Research started in 1972 as a visiting fellow and he became a regular research fellow a year later. He served the institution for three decades and half and before his superannuation from TIFR service as a senior professor in 2017, he held various positions as reader (1976–83), associate professor (1983–90) and professor (1990–93). After retiring from regular service, Roy continued his research at Saha Institute of Nuclear Physics as a DAE Raja Ramanna fellow of the Board of Research in Nuclear Sciences (BRNS) until 2012 and carries on his research at the Center for Astroparticle Physics and Space Science (CAPSS) of Bose Institute as a senior scientist of the Indian National Science Academy ever since. In between, he had several spells abroad as a visiting faculty and the institutions he has served include Oxford University, University of Texas at Austin, University of California, Riverside, University of Hawaii at Manoa, University of Helsinki and Kavli Institute for Theoretical Physics.