Davies was born Marion Cecilia Douras on January 3, 1897, in Brooklyn, the youngest of five children born to Bernard J. Douras (1857–1935), a lawyer and judge in New York City; and Rose Reilly (1867–1928). Her father performed the civil marriage of Gloria Gould Bishop. Her elder siblings included Rose, Reine, and Ethel. A brother, Charles, drowned at the age of 15 in 1906. His name was subsequently given to Davies’ favorite nephew, screenwriter Charles Lederer, the son of Davies’ sister Reine Davies.  The Douras family lived near Prospect Park in Brooklyn. The sisters changed their surname to Davies, which one of them spotted on a real-estate agent’s sign in the neighborhood. Even at a time when New York was the melting pot for new immigrants, having a British surname greatly helped one’s prospects – the name Davies has Welsh origins.  Educated in a New York convent, Davies left school to pursue a career. She worked as a model and posed for illustrators Harrison Fisher and Howard Chandler Christy. In 1916, Davies was signed on as a Ziegfeld girl in the Ziegfeld Follies.

After making her screen debut in 1916, modelling gowns by Lady Duff-Gordon in a fashion newsreel, she appeared in her first feature film in the 1917 Runaway Romany. Davies wrote the film, which was directed by her brother-in-law, prominent Broadway producer George W. Lederer. The following year she starred in three films – The Burden of Proof, Beatrice Fairfax, and Cecilia of the Pink Roses. Playing mainly light comic roles, she quickly became a film personality appearing with major male stars, making a small fortune, which enabled her to provide financial assistance for her family and friends.  In 1918, Hearst started the movie studio Cosmopolitan Productions to promote Davies’ career and also moved her with her mother and sisters into an elegant Manhattan townhouse at the corner of Riverside Drive and W. 105th Street. Cecilia of the Pink Roses in 1918 was her first film backed by Hearst. She was on her way to being the most infamously advertised actress in the world. During the next ten years she appeared in 29 films, an average of almost three films a year. One of her most known roles was as Mary Tudor in When Knighthood Was in Flower (1922), directed by Robert G. Vignola, with whom she collaborated on several films.

By the mid-1920s, however, Davies’ career was often overshadowed by her relationship with William Randolph Hearst and their social life at San Simeon and Ocean House in Santa Monica; the latter dubbed by Colleen Moore “the biggest house on the beach – the beach between San Diego and Vancouver”.  According to her own audio diaries, she met Hearst long before she had started working in films. Hearst later formed Cosmopolitan Pictures, which would produce most of her starring vehicles. Hearst’s relentless efforts to promote her career had a detrimental effect, but he persisted, making Cosmopolitan’s distribution deals first with Paramount, then Goldwyn, and then Metro Goldwyn Mayer. Davies herself was more inclined to develop her comic talents alongside her friends at United Artists, but Hearst pointedly discouraged this. Davies, in her published memoirs The Times We Had, concluded that Hearst’s over-the-top promotion of her career, in fact, had a negative result. Example: in 1929 Mr. Hearst purchased the Cameo Theatre, 934 Market Street, San Francisco. He then lavishly remodeled both the exterior and interior decor in a rosebud-hued Art Moderne motif, and renamed it The Marion Davies Theatre. From Hearst’s office windows further up Market Street, he could see pink neon letters constantly spelling out her name above the marquee. Hearst Metrotone Newsreels were included on the program, and these newsreels regularly touted Miss Davies’ social activities.

Hearst loved seeing her in expensive costume pictures, but she also appeared in contemporary comedies like Tillie the Toiler, The Fair Co-Ed (both 1927), and especially three directed by King Vidor, Not So Dumb (1930), The Patsy and the backstage-in-Hollywood saga Show People (both 1928). The Patsy contains her imitations which she usually did for friends, of silent stars Lillian Gish, Mae Murray and Pola Negri. King Vidor saw Davies as a comedic actress instead of the dramatic actress that Hearst wanted her to be. He noticed she was the life of parties and incorporated that into his films.  After seeing photographs of St Donat’s Castle in Country Life magazine, the Welsh Vale of Glamorgan property was bought and revitalized by Hearst in 1925 as a gift to Davies. Hearst and Davies spent much of their time entertaining, holding lavish parties with guests at their Beverly Hills estate. Frequent guests included, among others, Charlie Chaplin, Douglas Fairbanks, and a young John F. Kennedy. Upon visiting St Donat’s, George Bernard Shaw was quoted as saying: “This is what God would have built if he had had the money.”

The coming of sound made Davies nervous because she had never completely overcome a childhood stutter.[9] Her career continued, however, and she made several comedies and musicals during the 1930s, including Marianne (1929), Not So Dumb (1930), The Florodora Girl (1930), The Bachelor Father (1931), Five and Ten (1931) with Leslie Howard, Polly of the Circus (1932) with Clark Gable, Blondie of the Follies (1932), Peg o’ My Heart (1933), Going Hollywood (1933) with Bing Crosby, and Operator 13 (1934) with Gary Cooper. She was involved with many aspects of her films and was considered an astute businesswoman. Her career, however, was hampered by Hearst’s insistence that she play distinguished, dramatic parts as opposed to the comic roles that were her forte.

Hearst reportedly had tried to push Irving Thalberg to cast Davies in the title role in Marie Antoinette, but Thalberg gave the part to his wife, Norma Shearer. This rejection came on the heels of Davies having been also denied the female lead in The Barretts of Wimpole Street; Norma Shearer got that role too. Despite Davies’ friendship with the Thalbergs, Hearst reacted by pulling his newspaper support for MGM and moved Cosmopolitan Pictures to Warner Brothers. Davies’ films there included Page Miss Glory (1935), Hearts Divided, Cain and Mabel (both 1936), and Ever Since Eve (1937), her last film.  When Cosmopolitan Pictures folded, Davies left the film business and retreated to San Simeon. Davies would later state in her autobiography that after many years of work she had had enough and decided to devote herself to being Hearst’s “companion and confidante”. In truth, she was intensely ambitious, but faced the harsh reality that at the age of forty, after twenty years of effort, she had not won over the public, nor critics who were not under Hearst’s control. Decades after Davies’ retirement and death, however, the consensus among some critics is more appreciative of her efforts, particularly in the field of comedy.

In her later years, Davies was involved with charity work. In 1952, she donated $1.9 million to establish a children’s clinic at UCLA, which was changed to The Mattel Children’s Hospital in 1998. She also fought childhood diseases through the Marion Davies Foundation. Part of the Medical Center at UCLA is named the Marion Davies Clinic.  She suffered a minor stroke in 1956, and later underwent surgery on her jawbone for osteomyelitis. Twelve days after the operation, Davies fell in her hospital room and broke her leg. Davies made her last public appearance on January 10, 1960, on an NBC television special called Hedda Hopper’s Hollywood. Joseph P. Kennedy rented Davies’ mansion and worked from behind the scenes to secure his son John F. Kennedy’s nomination during the 1960 Democratic National Convention in Los Angeles. It was not long after that she was diagnosed with stomach cancer.

Davies died of stomach cancer on September 22, 1961, in her home in Hollywood, California.  Her funeral at Immaculate Heart of Mary Church in Hollywood was attended by 200 people and many Hollywood celebrities, including Mary Pickford, Charles “Buddy” Rogers, Mrs. Clark Gable (Kay Spreckels), and Johnny Weissmuller. She is buried in the Hollywood Forever Cemetery. Davies left an estate estimated at $20 million.