Doohan had been suffering from Alzheimer's disease, Parkinson's disease, diabetes, lung fibrosis and pneumonia, his agent Steve Stevens told the Canadian Broadcasting Corp.
One of his last public appearances was about a year ago, when he received a star on the Hollywood Walk of Fame.
Doohan was born in Vancouver, British Columbia, and grew up in Sarnia, Ontario. He was a captain in the Royal Canadian Artillery during WWII and participated in the Juno Beach invasion on D-Day.
He started his acting career on radio in 1946 and later moved over to TV, where he appeared in a number of CBC programs.
He became a page in science fiction pop culture history when he was tapped to play the Starship Enterprise's scrappy chief engineer Montgomery "Scotty" Scott on TV's original "Star Trek."
Doonan has requested he be cremated and his ashes launched into space by the same Houston-based aerospace company that shot the remains of Star Trek creator Gene Roddenberry into orbit following his 1991 death, E! Online reported.
The space funeral will occur "as soon as the next flight goes up," Stevens said.