Rainer, a former star of the Vienna stage who had been the oldest living actor to have won an Academy Award, died of pneumonia in London, her daughter said.
"She was an extraordinary woman who will undoubtedly leave an indelible print on the industry," her daughter Francesca Bowyer told Reuters. "She was a legend, she was my legend."
Rainer enjoyed a meteoric rise in Hollywood followed by an equally dramatic fall after she clashed with imperious Metro-Goldwyn-Mayer studio boss Louis B. Mayer over his iron-fisted control over her career.
After being assigned a succession of parts she did not like and being denied ones she wanted, Rainer contentiously parted ways with MGM, leaving Hollywood in 1938. She returned only briefly in 1943 to make a film for rival studio Paramount.
In a 1999 interview with the New York Times, Rainer recalled Mayer's parting threat: "We made you and we can kill you." She said she retorted: "Mr. Mayer, you didn't make me. God made me."
Rainer had an unhappy three-year marriage to playwright Clifford Odets, ending in 1940. When she became friends with Albert Einstein, Odets was said to have become so jealous that he used scissors to shred a photograph of the scientist.
Rainer wed British publishing executive Robert Knittel in 1945 and lived with him in London and Switzerland until his death in 1989. She lived alone in London afterward, with her two Oscars on a bookshelf in her study.
The statuette for "The Great Ziegfeld," in which she starred with William Powell and Myrna Loy, was the original. The one for "The Good Earth" was a replacement. She told the Telegraph in 2009 she gave that original to the workers who moved her from Switzerland to London after Knittel's death.
"I used it as a doorstop," Rainer said. "And it was bent."
Katharine Hepburn is the only other woman to win the best actress Oscar in consecutive years, for "Guess Who's Coming to Dinner" (1967) and "The Lion in Winter" (1968).
Rainer was born on Jan. 12, 1910, in Dusseldorf. She earned early success as a stage actress in Vienna, a protege of theatrical director Max Reinhardt, before dabbling in films. The rise of the Nazis in the early 1930s prompted Rainer, the daughter of a prosperous Jewish businessman who was an American citizen, to move to the United States.
'THE NEXT GARBO'
Rainer was an accomplished stage and screen actress when an MGM talent scout spotted her and told Mayer she would become "the next Garbo," referring to incandescent Swedish film superstar Greta Garbo, who was five years older than her.
Rainer replaced Loy opposite Powell in her first Hollywood film, "Escapade" (1935), then was cast again with Powell in the musical "The Great Ziegfeld," delivering an Oscar-winning performance as Ziegfeld's first wife.
For the epic film adaptation of the Pearl S. Buck novel "The Good Earth," Irving Thalberg, MGM's production chief, had wanted to cast Chinese-American actress Anna May Wong opposite Paul Muni, a white actor playing a Chinese farmer. But Thalberg was blocked from doing so because of Hollywood's ban at the time on on-screen interracial relationships.
Rainer got the role - over Mayer's opposition - of the long-suffering Chinese peasant wife, and won her second straight Oscar. That made Rainer the first actress to win multiple Academy Awards.
But things were souring at MGM, and she lamented that she was treated as merely a "tool in a big mechanical factory."
She appeared again with Powell in "The Emperor's Candlesticks" (1937), with Spencer Tracy in "Big City" (1937), with Melvyn Douglas in "The Toy Wife" (1938), and headed an ensemble cast in "Dramatic School" (1938). She starred as the wife of composer Johann Strauss in "The Great Waltz" (1938) before turning her back on Hollywood.
She returned to cinema at the age of 86 after an absence of more than half a century in the 1997 European film "The Gambler."
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