Mercedes appeared in a magazine advertisment for Rheingold Beer in 1950.
Mercedes was married twice; to William Fifield from 1941 to 1946, and to Fletcher Markle, from 1950 to 1962.
Mercedes was given a cowboy hat as a gift by Gary Cooper, and she wore it in filming Giant (1956).
Mercedes published her autobiograpy, "The Quality of Mercy: An Autobiography" in 1981.
Mercedes had one son, John Lawrence Fifield, who killed himself, his wife and children in 1987.
Mercedes was nominated for the Tony Award in 1972 for Best Supporting or Featured Actress in a Drama for "The Love Suicide at Schofield Barracks."
Mercedes fought numerous times with co-star Joan Crawford, who was jealous of her youth and attractive looks, on and off the set of Johnny Guitar (1954). After one fight, Crawford took Mercedes' clothes and scattered them down an Arizona highway near the filming location, and the film crew as well as Mercedes went to pick them up.
Mercedes did the voice-over work of the demon Pazuzu in The Exorcist (1973), and her name was left out of the credits, even though she had been promised by director William Friedkin it would be listed. An appeal to the Academy of Motion Pictures resulted in her name being restored in the 2000 version of the film released both in theaters and on DVD.
Mercedes worked with Orson Welles in his Mercury Theater and with Agnes Moorehead there. She later played the nemesis of Moorehead's Endora character, Carlotta, on the Bewitched episode "Darrin Gone! and Forgotten?"
Mercedes did 25 episodes of CBS Radio Mystery Theater between 1974 and 1978.
Mercedes won 2 Golden Globes in 1950, one for Best Supporting Actress in All The King's Men (1949) and the other for Most Promising Newcomer.
Mercedes has two stars on the Hollywood Walk Of Fame, one for motion pictures at 1722 Vine Street and one for television at 6243 Hollywood Boulevard.
Mercedes won the Best Supporting Actress Oscar in 1950 for All The King's Men (1949) and was nominated for it again in 1957 for Giant (1956).
Mercedes was 5 feet 3 inches tall.
Mercedes: If I had taken one-tenth of all the prescriptions prescribed for me, I would be long since dead. I never did take much medicine.
Mercedes: I believe in joy, but I believe in the flip-side, agony.
Mercedes: Alcohol is a very patient drug. It will wait for the alcoholic to pick it up one more time.
Mercedes: A rich man can afford to be generous to many.
Mercedes: At awards time, The Exorcist was nominated in 11 categories, everybody but the janitor was up for an Oscar. There was no category for what I did.
Mercedes: One of the most destructive things in my life was the kind of parts I played in pictures. I studied Shakespeare and the classics, and I end up shooting Joan Crawford and killing a horse that Elizabeth Taylor was in love with. I'm serious. I played the worst harridans, the most hard-bitten women, the absolute heavies, and it just about did me in.