Early career

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 two films – The Burden of Proof 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.

Hearst and Cosmopolitan Pictures

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 with 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."

Sound Films

The coming of sound made Davies nervous because she had never completely overcome a childhood stutter. 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 MGM production boss 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; that role going to Norma Shearer as well. Despite Davies' friendship with the Thalbergs, Hearst reacted by pulling his newspaper support for MGM and moved Cosmopolitan Pictures distribution 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 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.

Relationship with William Randolph Hearst

Publishing mogul William Randolph Hearst and Davies lived as a couple for decades but were never married, as Hearst's wife refused to give him a divorce. At one point, he reportedly came close to marrying Davies, but decided his wife's settlement demands were too high. Hearst was extremely jealous and possessive of her, even though he was married throughout their relationship. Lita Grey, the second wife of Charlie Chaplin, wrote four decades later that Davies confided with her about the relationship with Hearst. Grey quoted Davies saying:

God, I'd give everything I have to marry that silly old man. Not for the money and security—he's given me more than I'll ever need. Not because he's such cozy company, either. Most times, when he starts jawing, he bores me stiff. And certainly not because he's so wonderful behind the barn. Why, I could find a million better lays any Wednesday. No, you know what he gives me, sugar? He gives me the feeling I'm worth something to him. A whole lot of what we have, or don't have, I don't like. He's got a wife who'll never give him a divorce. She knows about me, but it's still understood that when she decides to go to the ranch for a week or a weekend, I've got to vamoose. And he snores, and he can be petty, and has sons about as old as me. But he's kind and he's good to me, and I'd never walk out on him.

By the late 1930s, Hearst was suffering financial reversals. After selling St Donat's Castle, Davies bailed him out by writing out a check for $1 million. Hearst died on August 14, 1951.

The California State Parks staff at Hearst Castle report at the time of Hearst's death, 51% of his fortune had been bequeathed to Davies.