- In 1940, during WWII, her son John, who was in the military, was reported missing and presumed dead. She and her husband had no further news about him for weeks until they found out he had been captured and was still alive. After the war ended in 1945, he was home within a few weeks.
- Considered by many critics to be one of the greatest actresses of the 20th century.
- First British actress to appear on a postage stamp
- Dame Sybil's ashes are buried in Westminster Abbey. Plot - The Nave (South Choir Aisle).
- Both she and husband Sir Lewis Casson took a keen interest in politics and were supporters of the Popular Front government in Spain during the Spanish Civil War, which was fighting against the Nazi-supported forces of Francisco Franco. She joined Emma Goldman, Rebecca West, Fenner Brockway and C.E.M. Joad in establishing the Committee to Aid Homeless Spanish Women and Children. Both she and Lewis held strong politically leftist beliefs and were active members of the Labour Party, to the extent that when the 1926 general strike cut short the run of "Saint Joan", they remained strongly on the side of the strikers. On a trip to South Africa in 1928, she and Lewis aggressively fought segregation so black theatergoers could experience their presentations.
- George Bernard Shaw wrote his play "Saint Joan" specifically for her, and she performed it in its original production.
- Sybil and husband Sir Lewis Casson were involved in the early days of British Equity, The Arts Council, The Old Vic, the National Theatre, Chichester Festival Theatre and, of course, the Thorndike Theatre in Leatherhead, which has since closed. Both Dame Sybil and Sir Lewis died at the ages of 93. Lewis died in May of 1969, a few months before the opening of The Thorndike Theatre in September.
- She was awarded the DBE (Dame Commander of the Order of the British Empire) in the 1931 King's Birthday Honours List and became a Companion of Honour (CH) in the 1970 Queen's Birthday Honours List for her services to drama.
- Was nominated for Broadway's 1957 Tony Award as Best Actress (Dramatic) for "The Potting Shed."
- She severely damaged her voice on a tour of the US presenting Shakespeare plays in 1905. Vocal problems would continue to plague her for the rest of her career.
- She was superstitious about washing her stage costumes, and would not allow anyone else to touch them.
- She was portrayed by Sian Thomas in the original production of the play "Fram", by Tony Harrison , which premiered at the National Theatre, London, UK in April 2008.
- Both she and her brother Russell Thorndike appeared in films starring and directed by Laurence Olivier: she appeared in The Prince and the Showgirl (1957) while her brother appeared in Henry V (1944), Hamlet (1948) and Richard III (1955).
- She played the wife of her real life brother Russell Thorndike in Macbeth (1922).
- A memorial service was held for her at Westminster Abbey, London on 2 July 1976.
- She was superstitious about the colour green and would not go on stage wearing anything in green.
- A cousin of actress Brenda Dunrich, whose mother Hilda (1881-?) was a Thorndike.
- A socialist, pacifist, supporter of the welfare state and the Arts Council. An opponent of nuclear weapons and the death penalty.
- She was married for 60 years and an actress for 65 years.
- Sister of actor Russell Thorndike and actress Eileen Thorndike. A fourth sibling Francis Herbert Thorndike (1894-1917) died as a pilot in the war.
- 1966: Created an honorary DLitt of the University of Oxford.
- Grandmother of Glynis Casson and of theater scholar Diana Devlin whose parents are Mary Casson and William Devlin.
- Son Christopher Casson.
- She started off as a concert pianist but her wrists became weak when she was 17 and she had to give it up and turned to acting.
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