Kate Winslet was born in Reading, Berkshire, England, the daughter of Sally Ann (née Bridges), a barmaid, and Roger John Winslet, a swimming-pool contractor. Both of her parents were also actors. Her maternal grandparents, Linda (Plumb) and Archibald Oliver Bridges, founded and operated the Reading Repertory, and her uncle, Robert Bridges, appeared in the original West End production of Oliver! Her sisters are Beth Winslet and Anna Winslet, also actresses.

Winslet, raised as an Anglican, began studying drama at the age of eleven at the Redroofs Theatre School, a co-educational independent school in Maidenhead, Berkshire, where she was head girl and appeared in a television commmercial for Sugar Puffs cereal, directed by Tim Pope. Throughout her adolescence, she was severely bullied for being overweight and having exceptionally large feet (which she inherited from her mother).

Winslet's career began on television, with a co-starring role in the BBC children's science fiction serial Dark Season in 1991. This was followed by appearances in the made-for-TV movie Anglo-Saxon Attitudes in 1992 and an episode of the medical drama Casualty in 1993, also for the BBC.

Her film career took off with praise and recognition in 1994 when she starred in a joint leading role, as Juliet Hulme in director Peter Jackson's critically acclaimed Heavenly Creatures, playing a vivacious and imaginative teen who helps her best friend (played by Melanie Lynskey), murder her mother when they are not allowed to be together.

This role was followed by the successful film Sense and Sensibility (co-starring Emma Thompson), which made her well-known, especially in the UK. Winslet became famous worldwide after the 1997 release of Titanic, a massive hit which holds the record as highest-grossing film in history (not accounting for inflation) at more than US$1 billion in box-office worldwide. It went on to win 11 Academy Awards.

She was also originally cast as Nola Rice in Woody Allen's movie, Match Point, but declined. She stated she had just finished two movies and wanted to spend time with her kids. The part went to Scarlett Johansson.

Winslet has been regarded as something of a critics' darling, generally receiving positive reviews for every one of her films. Despite Titanic's success, she has continued making lower-budget, independent films, including Hideous Kinky and Holy Smoke!; her roles in these smaller, more artistic films appear to be one of choice—she turned down the lead in Shakespeare in Love to make Hideous Kinky; ironically, she appeared as Ophelia in Kenneth Branagh's version of Hamlet.

She was even considered for the role of Satine in "Moulin Rouge!". Throughout her career, she has often turned down other roles in high profile films, such as Anna Anna and the King and Éowyn in The Lord of the Rings film trilogy. She has also taken several roles in studio "period dramas" like Quills, Titanic and Finding Neverland. For a time, she was given the nickname "Corset Kate".

In 2005, Winslet appeared in a television commercial for American Express. As part of the "My Life, My Card" campaign, the ad shows Winslet strolling around Camden Lock, in London, as she makes references to all the events that have happened to her film characters: going to prison for murder (Heavenly Creatures), being penniless and heartbroken (Sense and Sensibility), almost drowning (Titanic), losing her mind (Iris), having her memory erased (Eternal Sunshine of the Spotless Mind), and being in Neverland (Finding Neverland).

During the ad, she is shown holding items relating to her films; during the reference to Sense and Sensibility she thumbs through a copy of the book, and when she references Finding Neverland, she's holding a hook. When Winslet talks about nearly drowning at age 20 in Titanic, she is walking over a bridge with water underneath it, in reference to the iceberg and water seen in the film.

Winslet also appeared in an episode of BBC's comedy series Extras in August 2005, as a satirical version of 'herself'. She memorably told Andy and Maggie, the two characters who star in the series, that she was doing a film about the Holocaust because she was tired of losing out on Oscars, as at the time she had been nominated four times, and that everyone who does a film about the Holocaust wins an Oscar.

She also (while dressed as a nun) was shown giving phone sex tips to the romantically challenged Maggie. Ricky Gervais (who is a native of the same town as Winslet, Reading, as is her husband Sam Mendes) later said on NPR that she was his favorite guest star.

Her performance in the episode did lead to her being nominated for an Emmy Award for Outstanding Guest Performance in a Comedy Series, but she did not win.

As of March 2007, Leonardo DiCaprio signed to co-star in Revolutionary Road with Winslet as Frank and April Wheeler, a 1950s couple who appear content on the surface but are withering internally. The film will be the first to reunite the notable duo, who have remained close since their first pairing in Titanic.