Garai was born in Hong Kong to British parents. Her mother, Janet A. (née Brown), worked as a journalist, and her father, Adrian E. R. Garai, is a banker. Her first name is the female version of Romulus, one of the founders of Rome. Garai's great-grandfather, Bernhard "Bert" Garai, a Hungarian Jewish immigrant, founded the Keystone Press Agency, a photographic agency and archive, in London in the early twentieth century.
Garai is the third of four siblings. She moved to Singapore at five, before her family returned to Wiltshire in England when she was eight. She attended an independent boarding school, Stonar School in Wiltshire, and later moved at sixteen to London to attend the City of London School for Girls, where she completed 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 eighteen, where she was spotted by an agent who signed her to play the younger version of Dame Judi Dench's character in the BBC Films/HBO co-production for television, The Last of the Blonde Bombshells.
After A-levels, she studied English Literature at Queen Mary, University of London, before transferring and graduating with a first class degree from the Open University. She originally intended only to focus on her studies but later began acting full time during the summer holiday. She is a former model.
Acting career Garai's first professional acting role was in the 2000 BBC-HBO TV film The Last of the Blonde Bombshells, where she played Judi Dench's character as young. She then landed a part in the BBC 26-part television series Attachments (2000–2002). 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. 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. Her performance received mixed reviews – many critics felt let down after her previous impressive turns. Later that same year Vanity Fair was released. In Vanity Fair, she played Amelia Sedley. Co-starring Reese Witherspoon, Jim Broadbent, and James Purefoy, the movie was based on the 19th century novel by William Makepeace Thackeray and it was directed by Mira Nair. The movie received mixed reviews.
In 2005, Garai received another BIFA nomination, this time for their Best Supporting Actress award, for her performance as Siobhan in the independent film Inside I'm Dancing. Her portrayal earned her the British Supporting Actress of the Year award from the London Film Critics Circle. Also in 2005, she starred in a two-part drama made for television, entitled The Incredible Journey of Mary Bryant. While critics hailed it as "pleasingly old-fashioned adventure," it was her performance that won the most admiration and earn her two nominations: Best Lead Actress in Television from the Australian Film Institute and Most Outstanding Actress in a Drama Series from the Logie Awards. As The Observer noted: "As for the tireless Garai, she once again demonstrated an instinctive understanding of the vital difference between overperforming and overacting."
She can be seen in Kenneth Branagh's film adaptation of William Shakespeare's As You Like It (2006), as Celia. The film was released in some European cinemas before being broadcast in 2007 on HBO cable television in the U.S. In 2009, it opened in theatres in Mexico.
In 2007, Garai starred as Angel Deverell in François Ozon's Angel. The Independent named her one of the actresses of the year for her performance in Angel. Garai was also nominated for the Prix Lumiere award. (the French equivalent of the Golden Globes), as Best Female Newcomer for Angel, making her the first British actress to be nominated for a Prix Lumiere.
In 2007, she also starred in the Oscar-nominated movie Atonement as the teenaged Briony Tallis. Co-starring Keira Knightley, James McAvoy, Vanessa Redgrave, and Brenda Blethyn, the movie went on to receive seven Academy Award nominations, including Best Picture. Garai earned a Best Actress nomination from the Evening Standard British Film Awards for her performance. She also appeared in two Royal Shakespeare Company productions: as Cordelia in King Lear and as Nina in The Seagull, starring alongside Ian McKellen, Frances Barber, Sylvester McCoy, Jonathan Hyde, and William Gaunt. The run, which toured the world, went into residence in the New London Theatre where it ended mid-January 2008. She received rave reviews, especially as Nina in The Seagull, The Independent calling her a "woman on the edge of stardom," and This Is London calling her "superlative", and stating that the play was "distinguished by the illuminating, psychological insights of Miss Garai's performance." She reprised her role as Cordelia in a televised version of King Lear.
In 2008. she appeared in the feature film The Other Man alongside Liam Neeson, Laura Linney, and Antonio Banderas. Garai next starred in Stephen Poliakoff's World War II thriller Glorious 39, alongside Julie Christie, Jenny Agutter, Bill Nighy, Christopher Lee, and Eddie Redmayne. The movie had its world premiere at the Toronto Film Festival.
In 2009, she played the title role in a television adaptation of Jane Austen's Emma, a four-hour miniseries that premiered on BBC One in October 2009, co-starring Jonny Lee Miller and Sir Michael Gambon. Garai was nominated for a Golden Globe for her performance. Emma then appeared on American television as part of PBS' Masterpiece Classic anthology series, airing in most U.S. markets over three consecutive Sunday evenings during January and February 2010. American actress Laura Linney, who had co-starred with Garai in The Other Man, was the presenter for Masterpiece Classic during the anthology's "season" (US)/"series" (UK) at the time, and she introduced each of the three installments that featured her former co-star.
She was attached to play American First Lady Jacqueline Kennedy in Flying Into Love; she will also appear in Hettie MacDonald's feature Nova Scotia, in which she plays a character called Lucy Hay; and also in director Gareth Bryn's Driven, which is written by Catrin Clarke. Garai has also signed to play the female lead in I Was Bono's Doppelgänger, alongside Charlie Cox and another former co-star, Bill Nighy. Filming was set to begin in August 2009.
In 2009, The Sunday Times Magazine named her as one of Britain's Rising Stars alongside Matthew Goode, Andrea Riseborough, Hugh Dancy, Eddie Redmayne and others. In January of that year she travelled to the Syrian-Iraqi border to make a short film titled No Man's Land for the UNHCR, highlighting the plight of 800 Palestinian refugees living in the Al-Tanaf refugee camp. Of her visit to the refugee camps Garai states, "My trip to a refugee camp in Syria destroyed any hope that the horrors of Iraq might end, or that we are doing enough to help its victims." Garai has been hailed by her Glorious 39 director Stephen Poliakoff as "the next Kate Winslet" and someone who will "dominate British cinema" in the future.
In 2011, Garai starred in the four-part BBC drama The Crimson Petal and the White based on the novel by Michel Faber. She has commented on her racy part of Sugar, a 19th century mistress, that "standing in pants and suspenders, waiting for someone to call action, is pretty cringy ... By the end everyone on the set was like, 'Please just put it away.'" In 2011 she played Bel Rowley in the TV drama The Hour leading with Dominic West and Ben Whishaw, and the stage play The Village Bike at the Royal Court.
Garai has signed on to star alongside actress Anne Hathaway and Jim Sturgess in Lone Scherfig's One Day. She will also play the part of a junkie single mother in the independent British film Junkhearts with Eddie Marsan and Tom Sturridge,.