Zellweger was born in in Katy, Texas. She is the daughter of Emil Eric Zellweger (a Swiss immigrant) and Kjellfrid Iren Andreassen (a Norwegian immigrant of Sami origin from Tromso), and graduated from Katy High School in Katy, a suburb of Houston in 1987. She was a cheerleader and gymnast, and participated in the drama club. After high school, she went to the University of Texas at Austin to major in English. Though she took a drama class only because she needed a fine arts credit to complete her degree, it reminded her of how much she loved acting. During this time, she supported herself by taking a series of waitressing jobs in Austin, Texas.

After Zellweger graduated from college she moved to Los Angeles to pursue a career in acting. She won supporting roles in films such as Dazed and Confused, Reality Bites, Empire Records, The Whole Wide World, 8 Seconds, and Return of the Texas Chainsaw Massacre.

Zellweger first caught the attention of audiences around the world with her breakout role in the 1996's Jerry Maguire, where she played the romantic interest of Tom Cruise's character. Since then Zellweger has won acclaim in roles such as One True Thing opposite William Hurt and Meryl Streep and in Neil LaBute's Nurse Betty opposite Morgan Freeman. The role garnered the actress her first of three Golden Globe Awards.

However it wasn't until her role in Bridget Jones's Diary that she caught the eye of the Academy of Motion Picture Arts and Sciences. For her performance as a lonely, British singleton in the 2001 film, Zellweger received her first Best Actress Academy Award nomination. When cast as this role see was not highly liked by the public, told she was too skinny to play the chubby Bridget, but she managed to gain weight for the film.

In 2002, she starred with Michelle Pfeiffer in White Oleander and in Rob Marshall's Best Picture winning film Chicago opposite Catherine Zeta Jones, Richard Gere, Queen Latifah, and John C. Reilly. Zellweger earned her second Academy Award nomination as Best Actress, as well as the Screen Actors Guild and Golden Globe Award. In 2004, Zellweger finally received an Academy Award, this time as Best Supporting Actress in Anthony Minghella's Cold Mountain opposite Jude Law and Nicole Kidman.

Zellweger has since starred in the sequel to Bridget Jones' Diary in Bridget Jones: The Edge of Reason, lent her voice to the animated feature Shark Tale, and starred in the 2005 Ron Howard film Cinderella Man opposite Russell Crowe and Paul Giamatti. She recently finished filming on the biopic Miss Potter based on the life story of acclaimed author Beatrix Potter, with Emily Watson and Ewan McGregor. The film is due out by the end of the 2006.

On May 24, 2005, Zellweger received her star on the Hollywood Walk Of Fame.

On May 9, 2005, Zellweger married singer Kenny Chesney in a ceremony at the island of St. John. They had met in January at a tsunami relief benefit concert. Zellweger missed out on the engagement ring since the wedding was planned over a short span of time. On September 15, 2005, after only four months of marriage, they announced their plans for an annulment. Zellweger cited fraud as the reason in the related papers. After media scrutiny of her use of the word "fraud", she released the following statement: "(The term is) simply legal language and not a reflection of Kenny's character. I would personally be very grateful for your support in refraining from drawing derogatory, hurtful, sensationalized or untrue conclusions. We hope to experience this transition as privately as possible." The annulment was finalized in late December 2005.

Zellweger, together with Marc Forster, took part in the 2005 HIV prevention campaign of the Swiss federal health department. She's shown with the text "No intercourse without a condom" and "Semen or blood - never in the mouth".