"Titanic" star Winslet, 33, was Oscar-nominated five times in the past 13 years but had always returned home empty-handed.
Her best actress Oscar joins two Golden Globes, a Screen Actors Guild award, a British BAFTA and a handful of U.S. critics awards that Winslet has won this season.
"To the academy, thank you so much," she said. Winslet gave credit to her fellow best actress nominees -- Meryl Streep, Anne Hathaway, Angelina Jolie and Melissa Leo -- calling them "goddesses."
"I think we all can't believe we are in the category with Meryl Streep at all," Winslet said.
In addition to Winslet's acclaimed turn in "The Reader," she won plaudits this awards season for her performance as a disillusioned housewife in "Revolutionary Road." The two films were released only weeks apart.
Winslet told the Academy Award audience she would be lying if she said she hadn't practiced an Oscar acceptance speech before.
"I think I was probably 8 years old and staring into the bathroom mirror, and this would have been a shampoo bottle," she said gesturing to her golden statuette. "Well, it's not a shampoo bottle now!"
Winslet has said that portraying Hanna Schmitz, a former Nazi prison guard who embarks on an affair with a teen-age boy more than a decade later, was the most challenging role she had ever played because she had little in her own life to draw on.
"Just getting underneath the skin of such a complex person who had lived such an isolated life for so many different reasons, it was hard and quite lonely at times," she told Reuters in a recent interview.
Winslet found international fame as the co-star of the 1997 blockbuster "Titanic" but rather than embracing Hollywood's big time, she opted for smaller, art house movies often made in her native Britain.
In parts ranging from the quirky girlfriend in "Eternal Sunshine of the Spotless Mind" to the laundress in an 18th century insane asylum in "Quills," Winslet made a point of avoiding the cute blonde actress career trap.
She is a close friend of her "Titanic" co-star Leonardo DiCaprio, but it took the pair more than ten years to team up again, this time in the anti-romance "Revolutionary Road."
Winslet is married to Oscar-winning director Sam Mendes and has two young children. On Sunday she thanked all three for "letting me do what I love and who love me just the way that I am."http://www.reuters.com/news/entertainment