Born in London, Beckinsale is the daughter of actor Richard Beckinsale, who died from a heart attack in 1979, and actress Judy Loe. She has a paternal half-sister, Samantha, who is also an actress. Beckinsale's paternal great-grandfather was Burmese, and Beckinsale has said that she was "very oriental-looking" as a child.

Beckinsale attended the private Godolphin and Latymer School, an all-girls independent school in London. In her teens, Beckinsale twice won the W. H. Smith Young Writers' competition — once for three short stories and once for three poems. After a rebellious adolescence, including a period of anorexia and starting a smoking habit, she followed in the footsteps of her parents and began her acting career. Her first role was in One Against the Wind, a television film about World War II that was first aired in 1991. Having gained three language A levels, Beckinsale studied French and Russian literature at New College, Oxford, though she did not finish her degre