Bell was born in London, England, the daughter of Mina, a personal assistant and nurse, and Peter Bell, an architect. Her mother was Iranian and her father English. Her parents divorced and she and her mother moved to Los Angeles, California when she was three years old. She became a naturalized citizen of the United States at the age of twelve. A self-described "geek", Bell thrived in math and biology classes in school, and in junior high school once programmed a computer video game.
She acted in various television advertisements as a child. She attended the University of California, Los Angeles (UCLA) with an interest in becoming either a biomedical engineer or a physician but dropped out to become a full-time model.
