Johnsen joined the Monell Chemical Senses Center in 1980 and taught in the Department of Biology and College of Veterinary Medicine at the University of Pennsylvania.
His principle field of research was in the area of animal sensory physiology working with fish including salmon and sharks, alligators and birds.
Johnsen became a member of the Senior Executive Service in 1994 and was named Director of the USDA National Center for Agricultural Utilization Research in Peoria, Illinois.
He led an R&D enterprise with 100 Ph.D. scientists and 170 technical support personnel in research on materials science, bioprocessing, and genetic engineering that utilized agricultural commodities for industrial and consumer products.
In 2013 Johnsen was named president and CEO of the PRB Foundation, which supports innovation in healthcare and community well-being by providing grants to academic and clinical research organizations.