Garai was born in Hong Kong, the daughter of Janet, a journalist, and Adrian Garai, a high-ranked bank manager. She relocated to Singapore at five before her family returned to Wiltshire in the United Kingdom when she was eight. Garai's father is of Jewish Hungarian descent;her great-grandfather was Bert Garai, the founder of the Keystone Press.
She attended an independent boarding school, at Stonar and later moved at sixteen to London to attend the City of London School for Girls where she ended up finishing off her A-levels. She was fond of drama and appeared in school plays, and also with the National Youth Theatre up until the age of 18, where she was spotted by an agent who whisked her away to play the younger version of Judi Dench's character in a television production called The Last of the Blonde Bombshells.
After A-levels, she studied English Literature at Queen Mary, University of London; she originally intended to only study but decided to do acting on the side during the summer holidays.
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.Many critics hailed her as the next Julie Christie taking into account not only Romola's acting talent but also her uncanny resemblance to the screen legend.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 – many critics felt let down after her previous impressive turns. Later that same year Vanity Fair was released.