"Day had been in excellent physical health for her age, until recently contracting a serious case of pneumonia, resulting in her death", the foundation said in a statement.
The Academy of Motion Picture Arts and Sciences led an outpouring of affection and sadness from Hollywood, hailing Day's "humour, extraordinary talent and kind heart". She had her first hit recording, "Sentimental Journey", with the group, in 1944. She lived in Monterey, California, devoting much of her time to the Doris Day Animal Foundation, and battling sensational reports that she'd become a broke recluse and a "bag lady".
Beyond her life on the big screen, Day released 29 studio albums of music between 1949 and 2011. The Hollywood legend opened up in her book about her tumultuous personal life, divorces, and financial hardship. Some of her husbands felt they played second fiddle to them. Day famously said after learning her real age when the Associated Press revealed her birth year had been reported incorrectly.
Day also suffered from panic attacks in the early '50s, what she later described as 'tantamount to a nervous breakdown'.
Born in Cincinnati, Ohio in 1922, Day went on to become a star of the Classical Hollywood era, beginning with her role in 1948's Romance On The High Seas. She abandoned that dream after her right leg was broken in an auto accident. During a long convalescence, she immersed herself in the big-band sounds of Tommy Dorsey and the vocals of Ella Fitzgerald.
But what was her final public performance?
The Humane Society of the United States, of which The Doris Day Animal League is an affiliate, praised Day as a pioneer in animal protection.
'But I never did like it. She was one of the most popular and acclaimed singers and actors of her generation, scoring top ten records and box office hits on stage and screen. Part of it was her speaking voice, ambrosial and bell-like but also a little musky, like a mysterious elixir viewed through amber-colored glass. She began her professional singing career at the young age of 15.
Brown said later, 'I'd say that next to Sinatra, Doris is the best in the business on selling a lyric'. Everyone knows her megahit, "Que Sera, Sera", a sort of anthem about going whichever way the wind takes you, the flawless song to sing to a child who, in the way children are, is eager for all the answers.
In her autobiography, Day recalled her son Terry Melcher, telling her that the $20 million she had earned had vanished and she owed around $450,000, mostly for taxes.
Just a month ago, Day granted a phone interview to THR as she turned 97, summing up her legacy in film by saying, "I enjoyed working and always tried to do the best job I could with every role". And then she came to embrace the philosophy in the song. Her most famous movies include the Alfred Hitchcock thriller The Man Who Knew Too Much (opposite Jimmy Stewart) and Love Me or Leave Me with James Cagney.
Day turned nearly exclusively to comedies by the late "50s".
The star's last major hit in the USA, it is notable for the third verse - where Day's performance of the first and second verses superimposed, creating the illusion she's singing with herself.