The 34-year-old star separated from Mendes at an unspecified point earlier this year, Shillings law firm said. It wasn't clear whether divorce proceedings have begun. The pair have been married since 2003 and have two children.
In a statement e-mailed to journalists, the law firm said the split was "entirely amicable and is by mutual agreement."
"Both parties are fully committed to the future joint parenting of their children," the statement said.
A call and an e-mail seeking further comment were not immediately returned.
Winslet won the Academy Award for best actress for her dramatic role in "The Reader" in 2009. It was the first Oscar win for Winslet, who had been nominated five other times for her roles in "Titanic," "Sense and Sensibility," "Iris," "Eternal Sunshine of the Spotless Mind" and "Little Children."
Mendes, an acclaimed stage and film director, won the coveted best director Oscar for "American Beauty."
He directed his wife in "Revolutionary Road," a recent film about a crumbling marriage that reunited Winslet with her "Titanic" co-star Leonardo DiCaprio.
Reviews were largely favorable, but the film failed to set off the same box office fever that made "Titanic" one of the most successful films in history.
In "Revolutionary Road," Winslet portrayed a suburban housewife who became increasingly bored and desperate with her stay-at-home life. The film won Winslet a Golden Globe for best dramatic actress, and in her acceptance speech she thanked her husband for pushing her so hard.
"Thank you for directing this film, babe, and thank you for killing us every single day and really enjoying us actually being in such horrific pain," she said last year.
Mendes said at the time that directing his wife had been one of the best experiences of his life, although he said she liked to discuss the movie 24 hours a day while he preferred to do something else, like watch a baseball game, at night when work was done.
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Associated Press Writer Gregory Katz contributed to this report.
