Richardson was born in Marylebone, London, to the theatrical Redgrave family, the daughter of Actress Vanessa Redgrave and Director Tony Richardson, and the granddaughter of actors Sir Michael Redgrave and Rachel Kempson, Lady Redgrave. Actress Natasha Richardson (1963–2009) was her sister, and through her, she is the sister-in-law of actor Liam Neeson, and the aunt of Micheál and Daniel Neeson. She is the niece of actors Lynn Redgrave and Corin Redgrave and cousin of Actress Jemma Redgrave, who is five days younger than Richardson. Joely appeared as an extra at the age of three in the 1968 version of The Charge of the Light Brigade, directed by her father.