Cicely Mary Hamilton, the daughter of Danzil Hammill and Maude Piers, was born in Paddington as Cecily Mary Hammill. At the time of the birth, Hammill was a captain in the Gordon Highlanders. When Cicely was ten years old, her mother disappeared from her life. Although Cicely always refused to talk about the matter, it is believed her mother was committed to an asylum. With Hammill serving in Egypt, Cicely was brought up by foster parents.
After an education at a boarding school in Malvern, Cicely became a pupil-teacher. She disliked the work and soon found employment as an actress with a touring company. It was during this time she changed her name from Hammill to Hamilton. In 1897 Hamilton joined a Shakespearian company led by the American actor, Edmund Tearle. Over the next few years she appeared as Gertrude in Hamlet, Emilia in Othello and one of the witches in Macbeth.
Unable to obtain leading roles on the London stage, Hamilton decided to turn to writing. Her first play, The Traveller Returns, was performed at the Pier Theatre, Brighton, in May 1906. This was followed by Diana of Dobsons. The play was an immediate success and ran at the Kingsway, London, for 143 performances.
In 1908 Hamilton joined the Women's Social and Political Union. However, Hamilton disliked the autocratic way that Emmeline Pankhurst ran the organisation and after a few months left to join the Women's Freedom League. She was also a founder member of the Actresses' Franchise League and the Women Writers Suffrage League. Hamilton wrote two propaganda plays, How the Vote was Won (1909) and A Pageant of Great Women. She also joined with the composer, Ethel Smythe, to write March of the Women.
Hamilton's most important contribution to the feminist movement was the influential, Marriage as a Trade (1909). In the book Hamilton argued that woman were brought up to look for success only in the marriage market and this severely damaged their intellectual development.
On the outbreak of the First World War, Elsie Inglis, one of the founders of the Scottish Women's Suffrage Federation, suggested that women's medical units should be allowed to serve on the Western Front. With the financial support of the National Union of Women's Suffrage Societies (NUWSS), Inglis formed the Scottish Women's Hospitals Committee. Hamilton was one of the first women to join the organisation and in November 1914 helped to establish the 200 bed Auxiliary Hospital at Royaumont Abbey in France.
In the summer of 1916 Hamilton helped nurse soldiers wounded at the Battle of the Somme. This included treating 300 new patients in three days. Others who worked with her at Royaumont Abbey included Elsie Inglis, Louisa Martindale, Evelina Haverfield and Ishobel Ross.
In May 1917 Hamilton left the Scottish Women's Hospital Unit and joined the Women's Auxiliary Army Corps. After training in England, Hamilton returned to France where she took control of a postal unit. However, soon afterwards, she was asked to form a repertory company at the Somme. For the rest of the war Hamilton's company performed a series of plays for Allied soldiers fighting on the Western Front.
After the war Hamilton became a freelance journalist working for newspapers such as the Daily Mail, the Daily Mirror and the Daily Express. She was also a regular contributor to the feminist journal, Time and Tide where she campaigned for free birth control advice for women and the legalization of abortion.
Hamilton's autobiography Life Errant, was published in 1935. Other books written by Hamilton include Modern Italy (1932), Modern France (1933), Modern Russia (1934), Modern England (1938), Lament for Democracy (1940) and The Englishwoman (1940). Cicely Mary Hamilton died when 80 year-old.