After medical school, Kornfeld did an internship at Barnes Hospital in St. Louis, and spent 2 years (1963–1965)[3] as a research associate at the National Institute of Arthritis and Metabolic Diseases of the National Institutes of Health.[4] He then returned to Washington University where he has remained since, serving as the school's hematology division head for thirty years.[3] He and his wife Rosalind, with whom he collaborated scientifically,[5] were recruited to the faculty in 1966 alongside Phil Majerus by the University's Chairman of Medicine. Kornfeld was first an instructor of medicine,[6] was promoted to assistant professor, and eventually professor in 1972.[4] From 1991 to 1997, he served as the director of the Medical Scientist Training Program.[4]