Dame Agnes Sybil Thorndike CH DBE (24 October 1882 - 9 June 1976) was a British actress, and the sister of author Russell Thorndike.

This distinguished theatrical tragedienne will be remembered forever if only for the fact George Bernard Shaw wrote his classic "Saint Joan" work specifically for her. Her over six-decade career allowed for a gallery of sterling, masterful portrayals, both classic and contemporary, performing all over the world including Australia, New Zealand, South Africa, India and both Western and Eastern Europe. She was created a Dame of the British Empire in 1931, when her career was not quite half over, and in 1970 was made Companion of Honor to Queen Elizabeth. Born Agnes Sybil Thorndike on October 24, 1882 in Gainsborough, Lincolnshire, she was the daughter of a minor canon of Rochester Cathedral. She was the eldest of four children. One younger brother, Frank, was killed in WWI action, a tragedy that left her father inconsolable. He himself would die a few months later. Sybil first became a concert pianist until nerve injuries in her hands quickly altered her destiny. She, at brother Russell Thorndike's suggestion, decided upon acting. Russell would later become a novelist and his sister's biographer. Not a classic beauty by any stretch, Dame Sybil had sharp features, prominent cheek bones and a pronounced chin that gave her a rather severe look. At age 21 she and her brother began professionally in a touring company guided by actor-manager Ben Greet. She performed as Portia in a production of The Merchant of Venice in 1907 while touring in New York. The following year she met playwright George Bernard Shaw while understudying the role of Candida in a tour which was being directed by the writer himself. It was also during this tour that Sybil met and married actor Sir Lewis Casson and solidified one of the most respected personal and professional relationships the acting realm has known. She stayed with The Old Vic for five years (1914-1919) and in 1924 earned stardom as Shaw's Joan of Arc. Sybil's film career, unlike that of her esteemed contemporary Edith Evans (I), fell far short of expectations. Silent films recreated some of her finest theatrical experiences, including Lady Macbeth and, of course, Joan of Arc, but she would not evolve into a film star. She was sporadically utilized in later years as a flavorful character support and played a number of queens, dowagers and old crones with equal finesse. Such classic costumed fare would include Major Barbara (1941), Nicholas Nickleby (1947), Stage Fright (1950), Gone to Earth (1950), Lady with the Lamp, The (1951), Melba (1953), as Queen Victoria, and Prince and the Showgirl, The (1957) in which she managed to grab focus during her scenes with Laurence Olivier and Marilyn Monroe. In 1969, Sybil lent her name to the new theatre in Leatherhead, Surrey, which became The Thorndike. Despite her 87 years, she performed in the new play There Was An Old Woman in its first season. It was to be her final theatrical performance. Always a healthy, vigorous woman, she died of a heart attack on June 9, 1976 at the ripe young age of 93. She was survived by four children and a number of grandchildren and great-grandchildren.