Winslet was giddy and emotional as she accepted her trophy after five previous Oscar losses.
"I'd be lying if I haven't made a version of this speech before," the 33-year-old British actress said. "I think I was probably 8 years old and staring into the bathroom mirror and this (Oscar) would be a shampoo bottle. But it's not a shampoo bottle now."
She thanked her husband, director Sam Mendes, and their two children. And she also thanked her father, saying, "Dad, whistle or something 'cause then I'll know where you are." He whistled back from his seat at the Kodak Theatre.
"You just don't think that these dreams that seem so silly and so impossible could ever really come true," Winslet said backstage.
This is the first Oscar for Winslet, who's been nominated previously for her roles in "Titanic," "Sense and Sensibility," "Iris," "Eternal Sunshine of the Spotless Mind" and "Little Children."
But the British actress seemed a shoo-in to win this year: She gained Oscar momentum after snagging a best supporting actress award at the Golden Globes for her role as a former Nazi concentration camp guard in "The Reader," as well as best dramatic actress for her role as an unhappy housewife in husband Mendes' film "Revolutionary Road" that reunited her with "Titanic" co-star Leonardo DiCaprio.
Winslet, 33, received more recognition on the awards circuit for "The Reader." Her portrayal of Hanna Schmitz - a woman having a passionate affair with a teenager who encounters her again years later while she is on trial for Holocaust crimes - was raw and restrained, netting her additional trophies at the Screen Actors Guild Awards and British BAFTA awards.
The best-actress category was loaded with strong contenders: Meryl Streep was another front-runner with her 15th acting nomination as a prickly nun in "Doubt." Anne Hathaway played against her wholesome image as a toxic narcissist who leaves rehab to wreak havoc on her sister's wedding in "Rachel Getting Married." Angelina Jolie dug deep to portray a mother of a missing child in "Changeling." And Melissa Leo was powerful as mom who forges an unlikely friendship in "Frozen River."
Another Oscar loss would have tied Winslet with Deborah Kerr and Thelma Ritter as the only actresses to go zero-for-six.
Winslet, born into a family of working actors in Reading, England, got her first big break at age 17 when director Peter Jackson cast her in his 1994 film "Heavenly Creatures" as an obsessive teenager who helps in the murder of her best friend's mother.
She garnered her first Oscar nomination for supporting actress when Ang Lee chose her to play opposite Emma Thompson in his 1995 adaptation of Jane Austen's "Sense and Sensibility."
Her performance as the feisty rebellious socialite in James Cameron's 1997 "Titanic," the highest-grossing film of all time, catapulted her into international stardom. But despite her newfound fame, Winslet mostly shunned high-profile Hollywood films for edgier independent fare.