Born Sienna Rose Diana Miller in New York City, United States, Miller moved to London with her family when she was 18-month-old. Her mother, Josephine, is a South African-born British former model, who was a personal assistant to David Bowie and one-time manager of the Lee Strasberg Theatre and Film Institute in NYC. Her father, Edwin Miller, is an American banker turned dealer of Chinese art. Miller has a sister, Savannah, and two half-brothers, Charles and Stephen. Her parents divorced in 1987, and Miller moved to Parsons Green with her mother, who at the time was being treated for breast cancer. Miller's father remarried three times and moved to the Virgin Islands. Miller attended Heathfield School, a boarding school in Ascot, Berkshire where she also used to act and play Lacrosse. According to Celebrity Biographies – The Amazing Life Of Sienna Miller, she developed a "mischievous and rebellious side" when she became a teenager, often being "chastised" for smoking, drinking and flirting. After graduating at 18, Miller obtained a modelling gig for a German commercial for soda pop, and with the money she earned from it, she relocated in NYC and enrolled in a three-month course at the Lee Strasberg Theatre and Film Institute.