Roos was born in New York. He attended the University of Notre Dame in Indiana. After graduating, Roos moved to Los Angeles, where he pursued a career writing for television.
Roos supported himself by working as a word processor, and to this day jokes that he has that as a fall-back plan. Roos began his writing career when he had a friend of his impersonate an agent and represent him; a phone call led to a job with playwright Mart Crowley (The Boys in the Band), who at the time was Executive Producer of Hart to Hart. Roos went on to write for The Colbys, Nightingales, and other TV shows, before his spec scripts led to feature film writing assignments. His first major film was 1991's Academy Award-nominated Love Field, an interracial drama starring Michelle Pfeiffer and Dennis Haysbert.
Roos' work as the writer of the film Single White Female has earned him a permanent space in Hollywood movie trivia, since that title has entered the lexicon in reference to the film's psychopathic lead character who begins to take on her roommate's identity.
Roos' is well-known for his work writing strong and engaging female characters, a skill that has also been useful in his film direction, leading to Independent Spirit Award nominations for actors Lisa Kudrow, Christina Ricci, and most recently, Maggie Gyllenhaal. Roos himself has won a Best First Feature Independent Spirit Award, for The Opposite of Sex. Roos has polished or written the screenplay to many high-profile studio films, sometimes as uncredited script doctor.