He served as a resident in neurology at Tufts New England Medical Center and was Chief Resident in 1980. From 1980, he held a series of academic appointments at the Harvard Medical School, and achieved the rank of full professor there in 1996. In 2004, Kosik became the Harriman Professor of Neuroscience Research and Co-Director of the Neuroscience Research Institute at the University of California, Santa Barbara.