Jean Marsh, the sleek British actress who co-created Upstairs, Downstairs and won an Emmy for her performance as the prim and proper parlormaid Rose Buck on the acclaimed ITV drama, has died. She was 90.

Marsh died Sunday at her home in London from complications of dementia, filmmaker Michael Lindsay-Hogg told The New York Times.

Doctor Who historians know Marsh for her portrayal of Sara Kingdom, who ably assists William Hartnell’s Doctor in his fight against the mutant Daleks, on the third season of the BBC sci-fi series in 1965-66.

A year earlier, she had played King Edward’s French princess sister, Joanna, on the show’s “The Crusade” serial, then returned as the ruthless enchantress Morgaine for 1989’s “Battlefield” storyline.

On American TV shows, the London native starred as an android brought to an asteroid to keep a prisoner (Jack Warden) company on 1959’s “The Lonely,” the seventh episode of CBS’ The Twilight Zone,...