Dennis McKenna is an ethnopharmacologist and has studied plant hallucinogens for more than 20 years. In 1975, he co-authored the book Invisible Landscape, with his brother Terence McKenna. The book was based on their investigations of Amazonian hallucinogens in 1971. He also acted as co-star of his brother's book True Hallucinations, which further described their experiences while in the Amazon.

Dennis went on to study the botany, chemistry, and pharmacology of ayahuasca and oo-koo-he, which he wrote about for his thesis work. He earned his Master's degree in botany at the University of Hawaii in 1979 and his Doctorate in Botanical Sciences in 1984 from the University of British Columbia. Since that time, he has conducted extensive ethnobotanical fieldwork in the Peruvian, Colombian, and Brazilian Amazon.

In 1990, he joined Shaman Pharmaceuticals as Director of Ethnopharmacology and in 1993 he joined the Aveda Corporation as Senior Research Pharmacognosist. In 1998 Dennis co-founded the non-profit Institute for Natural Products Research (INPR) to promote research and scientific education with respect to botanical medicines and other natural medicines. He is a founding board member of the Heffter Research Institute, serves on the Advisory Board of the American Botanical Council, and has served as a board member for Botanical Dimensions.