|The House That Screamed (La Residencia; The Boarding School; The Finishing School) (1969) [Irene]: Dies of shock/blood loss (off-screen) when the killer chops off her hands in the attic; we hear the blow from below, and her body is shown afterwards when Lilli Palmer discovers her. (Thanks to Carlos)
Crucible of Terror (1971) [Millie]: Burned to death when she pushes Mike Raven into the furnace in his studio. At the time of her death, she's possessed by Me Me Lay's spirit, and has even taken on Me Me's disfigured appearance. (The make-up is so heavy that I can't be sure whether it was actually Mary or Me Me acting in the scene, but it's still Mary's character who dies.) (Thanks to Thomas)
Terror (1978) [Lady Garrick]: Decapitated with an sword by Patti Love's ghost, in the film-within-the-film sequence at the beginning of the movie.