Siân Elizabeth Phillips was born in Lichfield, Staffordshire, England in 1980 and is the youngest of three siblings. She took on a stage name to avoid confusion with fellow actress Siân Phillips, choosing Brooke after an English Civil War general Lord Brooke who was killed at Lichfield. She is the daughter of a police officer and a teacher. Her parents are Welsh. Brooke's early education was at The Friary School in Lichfield. She initially joined the Lichfield Youth Theatre at the age of 11 before becoming a member of the National Youth Theatre and subsequently training at the Royal Academy of Dramatic Art (RADA), from where she graduated in 2002.
