She is the elder daughter of David Birkin (a commander in the Royal Navy and a World War II war hero involved in various aspects of espionage) and actress Judy Campbell (née Judy Gamble), who made her name as a star of Noel Coward musicals. Birkin also is a member of a prominent British industrial family that made its fortune manufacturing lace in Nottinghamshire. A great-aunt, Winifred (Freda) May Birkin, later Mrs William Dudley Ward, was a mistress of the Prince of Wales (later Edward VIII of the United Kingdom, later HRH the Duke of Windsor). She is also related by blood or marriage to the actresses Penelope Dudley Ward and Rachel Ward as well as the film director Sir Carol Reed.
Charlotte Gainsbourg is Birkin's daughter from her relationship with Serge Gainsbourg (né Lucien Ginzburg), whom she met in 1968 and lived with until 1980. She also has a daughter, Kate Barry (born 1968), from her first marriage to John Barry (they separated in 1968 and later divorced), an English composer who wrote the theme to the James Bond movies. Birkin has another daughter, Lou (born 1982), by film director Jacques Doillon.