It would seem that 2004, the year of her 18th birthday, will be remembered as pivotal for Emmy Rossum due to her appearance in two very different films, The Day After Tomorrow (2004) and The Phantom of the Opera (2004). Emmy's performance in the latter film gained her a Golden Globe nomination.

Emmanuelle Grey Rossum was born in New York City, where she was raised by her single mother, Cheryl Rossum, a corporate photographer (she has only met her father a few times). Her mother is of Russian Jewish descent and her father has English and Dutch ancestry. After passing an audition at the Metropolitan Opera when she was 7 years old, Rossum performed in more than 20 operas in six different languages at Lincoln Center, alongside such figures as Plácido Domingo and Luciano Pavarotti. She was directed by Franco Zeffirelli in "Carmen." She left the opera when she entered her teenage years, as she had grown too tall to perform as a child. Emmy also appeared in a Carnegie Hall presentation of "The Damnation of Faust." She graduated from the Spence School, a private institution in Manhattan, in 1996 and then earned a high school diploma when 15 years old by taking online extension courses offered by Stanford University (Education Program for Gifted Youth). She later enrolled at Columbia University and studied art history and French.

In a change of venue, Emmy created the role of Abigail Williams in the daytime soap opera As the World Turns (1956) in 1997 and branched out in performances in the made-for-television movies Genius (1999) and The Audrey Hepburn Story (2000), in which she played the title character as a young teenager. Other television work included Snoops (1999), Law & Order (1990), and The Practice (1997).

Emmy made her theatrical feature debut in the indie film Songcatcher (2000), with her good friend Rhoda Griffis, which won the Special Jury Award for Outstanding Ensemble Performance at the Sundance Film Festival in January 2000. Rossum received an Independent Spirit Award nomination in the category of Best Debut Performance for her performance as an Appalachian orphan. She played an aspiring songwriter (the title character) in the romantic comedy Nola (2003). Cast as the ill-fated daughter of a small-business owner in Clint Eastwood's Mystic River (2003), she projected an aura of innocence that made her character's tragic death memorable and heartbreaking. This was her first major studio film.

After six months of filming her role as the fresh-faced but highly intelligent teenage damsel in distress The Day After Tomorrow (2004) in Montreal, she returned to New York and screen-tested for the role of Christine in The Phantom of the Opera (2004) in full costume and makeup, and was finally selected for the part by Andrew Lloyd Webber after singing for him at his home. Although she was surprised to be chosen ahead of many better-known and older actresses considered for the part, the combination of her vulnerable, fragile beauty and fine, classically trained singing voice ultimately proved that she was perfectly cast. In preparation for the role, she took ballet classes for two months and started polishing her singing. Emmy has commented that, in her approach to acting, she draws heavily upon her own experiences, so she visited locations in Paris and conjured up what she terms "past memories" to draw upon in making her performance emotionally realistic. She stood on the roof of the Opéra Garnier, where Christine sings "All I Ask of You," and went underneath the opera house, where there is actually a gloomy, dark lake. She studied Degas's paintings of ballerinas in the Musée d'Orsay to learn how to stand like one.

Her next project Poseidon (2006) was a mainstream effort, but since its release, she has been more true to advice she obtained from Sean Penn when making Mystic River (2003), that she should be picky and only accept roles that are fun to do, such as Dragonball Evolution (2009).