People Magazine reported on Saturday that she died in California with loved ones, citing a family spokesperson. No other details were immediately available, and representatives for Keaton did not immediately respond to inquiries from The Associated Press.
The unexpected news was met with shock around the world.
"She was hilarious, a complete original, and completely without guile, or any of the competitiveness one would have expected from such a star. What you saw was who she was…oh, la, lala!," Bette Midler said in a post on Instagram. She and Keaton co-starred in The First Wives Club.
Keaton was the kind of actor who helped make films iconic and timeless, from her "La-dee-da, la-dee-da" phrasing as Annie Hall, bedecked in that necktie, bowler hat, vest and khakis, to her heartbreaking turn as Kay Adams, the woman unfortunate enough to join the Corleone family.
Her star-making performances in the 1970s, many of which were in Woody Allen films, were not a flash in the pan either, and she would continue to charm new generations for decades.
She played a businessperson who unexpectedly inherits an infant in Baby Boom, the mother of the bride in the beloved remake of Father of the Bride, a newly single woman in The First Wives Club, and a divorced playwright who gets involved with Jack Nicholson's music executive in Something's Gotta Give.
Keaton won an Oscar for Annie Hall in 1978 and would go on to be nominated three more times, for Reds, Marvin's Room and Something's Gotta Give.
Keaton was born Diane Hall in January 1946 in Los Angeles and was drawn to theatre and singing while in school.
She dropped out of college after a year to make a go of it in Manhattan. Actors' Equity already had a Diane Hall in their ranks, and she took Keaton, her mother's maiden name, as her own.
She started on the stage as an understudy in the Broadway production for Hair, and in Allen' s Play It Again, Sam in 1968, for which she would receive a Tony nomination.
Keaton made her film debut in the 1970 romantic comedy Lovers and Other Strangers, but her big breakthrough would come a few years later when she was cast in Francis Ford Coppola's The Godfather, which won best picture and become one of the most beloved films of all time.
The 1970s were an incredibly fruitful time for Keaton thanks in part to her ongoing collaboration with Allen in both comedic and dramatic roles. She appeared in Sleeper, Love and Death, Interiors, Manhattan, Manhattan Murder Mystery and the film version of Play it Again, Sam.
Allen gave Keaton one of her most iconic roles in Annie Hall, the infectious woman from Chippewa Falls whom Allen's Alvy Singer cannot get over. The film is considered one of the great romantic comedies of all time, with Keaton's eccentric, self-deprecating Annie at its heart.
In the New York Times, critic Vincent Canby wrote, "As Annie Hall, Miss Keaton emerges as Woody Allen's Liv Ullman. His camera finds beauty and emotional resources that somehow escape the notice of other directors. Her Annie Hall is a marvellous nut".
Keaton and Allen were also in a romantic relationship, from about 1968, when she met him while auditioning for his play, until about 1974. Afterward they remained collaborators and friends.
"He was so hip, with his thick glasses and cool suits," Keaton wrote in her memoir.
"But it was his manner that got me, his way of gesturing, his hands, his coughing and looking down in a self-deprecating way while he told jokes."
She was also romantically linked to Al Pacino, who played her husband in The Godfather, and Warren Beatty who directed her and whom she co-starred with in Reds. She never married but did adopt two children when she was in her 50s: a daughter, Dexter, and a son, Duke.
"I figured the only way to realise my number-one dream of becoming an actual Broadway musical comedy star was to remain an adoring daughter. Loving a man, a man, and becoming a wife, would have to be put aside," she wrote in the memoir.
"The names changed, from Dave to Woody, then Warren, and finally Al. Could I have made a lasting commitment to them? Hard to say. Subconsciously I must have known it could never work, and because of this they'd never get in the way of achieving my dreams."
In the remake of Father of the Bride, Keaton and Steve Martin played the flustered parents to the bride which would become a big hit and spawn a sequel.
In Something's Gotta Give in 2003 she begins a relationship with a playboy womaniser, played by Jack Nicholson, while also being pursued by a younger doctor, played by Keanu Reeves.
Her character Erica Barry, with her beautiful Hamptons home and ivory outfits was a key inspiration for the recent costal grandmother fashion trend. It earned her what would be her last Oscar nomination and, later, she'd call it her favourite film.
Keaton continued working steadily throughout the 2000s, with notable roles in The Family Stone, as a dying matriarch reluctant to give her ring to her son, in Morning Glory, as a morning news anchor, and the Book Club films.
