Collins began acting at the age of two in the BBC series Growing Pains. As a teenager, Collins wrote a column (titled NY Confidential) for the British magazine Elle Girl. She has also written for Seventeen, Teen Vogue, and the Los Angeles Times magazines.She was chosen by Chanel to wear one of its gowns at the 2007 Bal des Débutantes (a charity ball) at the Hôtel de Crillon in Paris, which was featured on season 3 of the reality TV show The Hills. She was picked by Spain's Glamour magazine in 2008 as their "International model of the year" and appeared on the magazine's cover in August 2009. Collins covered the 2008 US Presidential campaign as a host on the Nickelodeon show Kids Pick the President. She won a 2008 Young Hollywood Award for "Newest Red Carpet Correspondent". She has appeared in two episodes of the television show 90210, including the 2008–2009 season finale. Collins was one of twenty women named by Maxim magazine as one of "The Hottest Daughters of Rock Stars" in 2009. Collins was in the 2009 film The Blind Side as Collins Tuohy, the daughter of Sandra Bullock's character Leigh Anne Tuohy. She played the warrior priest's daughter Lucy in the thriller Priest, opposite Paul Bettany. MTV Networks' NextMovie.com named her one of the "Breakout Stars to Watch for in 2011".
She co-starred in the 2011 action film Abduction with Taylor Lautner. Collins played Snow White in Mirror Mirror, an adaptation of the Snow White fairytale, co-starring Julia Roberts. Robbie Collin from British newspaper The Telegraph wrote, "She has an adorable, sensational, almost perfect face for cinema; think Audrey Hepburn with the eyebrows of Liam Gallagher. Her smile is the Platonic ideal of cheeky." She made her singing debut in the film, performing an English re-written cover of "I Believe (in Love)".