Hardwicke died Monday in a hospice in Chichester in southern England, the Conway van Gelder Grant talent agency said Wednesday.
The English actor, who took on the Watson role in the second year of the series after David Burke dropped out, was an unflappable counterpoint to Brett's brooding, nervy characterization of the detective.
Hardwicke played Watson from 1986-88 in "The Return of Sherlock Holmes," and also played the character in the later series from 1991.
"People said I seemed to be an older, graver Watson. That always worried me a bit," Hardwicke once confessed.
Arthur Conan Doyle, the creator of Sherlock Holmes, described Watson as the detective's "rather stupid friend," but that's not how Hardwicke played the role.
"I think Conan Doyle is one of the few writers who has created a fictional genius. And I think anyone is going to appear stupid, or seen to be a bit slow, by comparison," Hardwicke said in a television interview.
Hardwicke, born in London, was the son of the actors Cedric Hardwicke and Helena Pickard. His film debut was in an uncredited bit part in "A Guy Named Joe," a 1943 film starring Spencer Tracy.
Hardwicke and Brett, who died in 1995, were both in the National Theatre between 1964 and 1972 when the late Laurence Olivier was leading the company. "My God, how lucky we were," Hardwicke said.
"Shadowlands," a film about the unlikely romance of Oxford academic C.S. Lewis and the American Joy Davidson, gave Hardwicke his most acclaimed film role as Lewis' brother Warnie. He was also known for playing a character based on war hero British Army officer Pat Reid in the BBC drama "Colditz."
The actor also did voice work for video games including "Napoleon Total War" and "Lara Croft Tomb Raider: Anniversary."
He is survived by his wife, Prim Cotton, and two daughters by his previous marriage.
Funeral arrangements were not immediately announced.
