Liu began acting in 1989, after auditioning for a role in the University of Michigan's production of Alice in Wonderland during her senior year. Liu was cast in the lead role, although she had originally only tried out for a supporting part. Liu had small roles in films and TV (including The X-Files in "Hell Money" and Hercules: The Legendary Journeys in "The March to Freedom") before landing a role on Ally McBeal. Liu originally auditioned for the role of 'Nelle Porter' (played by Portia de Rossi), and the character 'Ling Woo' was later created specifically for her. Liu's part on the series was originally not meant to be regular but the enthusiastic audience response to the actress's 'feisty' Ling Woo secured Liu as a permanent cast member. It also earned her an Emmy nomination for Outstanding Supporting Actress in a Comedy Series, as well as a Screen Actors Guild Award nomination for Best Actress in a Comedy Series. Liu cemented her reputation playing bad girls by portraying "Pearl" the sadistic dominatrix/hitwoman for the Chinese mafia in the film Payback (1999).
Liu played Alex Munday in the Charlie's Angels film, alongside established Hollywood stars Drew Barrymore and Cameron Diaz. The film opened in November 2000 and was a hit, earning more than $125 million in the U.S., and a worldwide total of more than $264 million. The sequel, Charlie's Angels: Full Throttle, opened in June 2003 and was a box-office hit again, earning more than $100 million in the U.S., and a worldwide total of more than $259 million. In between the two films, Liu starred with Antonio Banderas in Ballistic: Ecks vs. Sever, a critical and box office failure.
Liu next played Rita Foster in Vincenzo Natali's Brainstorm (original title Cypher) in 2002. This was followed shortly after by her role as O-Ren Ishii, one of the major villains in Quentin Tarantino's 2003 film, Kill Bill. She won an MTV Award for "Best Movie Villain" for the part. Subsequently, Liu appeared on several episodes of Joey with Matt LeBlanc, who played her love interest in the Charlie's Angels films. She also had smaller roles as Kitty Baxter in the film Chicago, and as a psychologist opposite Keira Knightley in the thriller Domino. In 2006, she played leading lady and love interest to Josh Hartnett in the crime thriller Lucky Number Slevin. Other appearances include a cameo on the animated shows Futurama (as herself and/or robot duplicates thereof in the episodes "I Dated a Robot" and "Love and Rocket") and The Simpsons (on the season sixteen episode "Goo Goo Gai Pan"), and a guest host on an episode of the NBC sketch show Saturday Night Live in 2000 (musical guest: Jay-Z).
