McKenna’s research led to the development of natural products for Aveda Corporation as well as greater awareness of natural products and medicines. He has authored numerous scientific articles and books. He co-authored The Invisible Landscape with his brother Terence and Botanical Medicines: the Desk Reference for Major Herbal Supplements with Kenneth Jones and Kerry Hughes. McKenna spent a number of years as a senior lecturer for the Center for Spirituality and Healing, part of the Academic Health Center at the University of Minnesota, Twin Cities. He became a senior research scientist for the Natural Health Products Research Group at the British Columbia Institute of Technology in the Vancouver area.