Dorman, who had a history of heart problems, was found dead in his car Friday in Orange County, E! Online reported.
Police said they do not believe his death is suspicious, TMZ reported.
The Saint Louis, Mo., native joined Iron Butterfly in 1967 and played with the band through its multiple incarnations on and off throughout the years, E! Online said.
More than 30 million copies of "In-A-Gadda-Da-Vida," featuring a 17-minute epic title track, have been sold over the years.