Kennedy, a Jacksonville, Fla., native, passed Saturday morning at Baptist Medical Center South, The (Jacksonville) Florida Times-Union reported.
Kennedy checked in to the hospital earlier this week for palliative care, which is treatment of pain aimed to comfort a patient rather than the treatment of a condition.
He was best known for his book "The Klan Unmasked," which exposed secrets of the Ku Klux Klan, published in the 1950s. In the book, Kennedy wrote about his infiltration of the KKK and a neo-Nazi group called the Columbians in 1946-47.
Although Kennedy admitted the book was actually based on another undercover man's experiences with the groups, a review of records show Kennedy worked as an undercover journalist to unmask the Klan and the Columbians.