It was during her first break from University that Garai landed a part in a BBC-produced television series called Attachments. It was this production that prompted her to make the decision to stop her education and concentrate solely on her acting career.

Garai's first major film role was in 2002's Nicholas Nickleby. She played Kate Nickleby, a supporting role, in the well-reviewed film. The entire cast was widely recognized for their work and were awarded Best Ensemble by the National Board of Review. In 2003's I Capture the Castle, she played 17-year-old Cassandra Mortmain. She received glowing praise for her work and the film scored 80% at Rotten Tomatoes/ Her performance earned her a nomination for a Most Promising Newcomer award from the British Independent Film Awards. Dirty Dancing: Havana Nights (2004) was Garai's biggest critical flop to date, scoring only 23% at Rotten Tomatoes (though it went on to make $27 million worldwide). Her performance received mixed reviews