She was born Sarah Kemble in Brecon, Wales, the eldest daughter of Roger Kemble, an actor-manager whose travelling company included most members of his family. Sarah's brothers, Charles Kemble, John Philip Kemble and Stephen Kemble were all actors.
Acting was only just becoming a respectable profession for a woman. In 1773, Sarah married William Siddons, another member of the company, and made her debut at Drury Lane two years later, as Portia in The Merchant of Venice. By the time she next appeared there in 1782, she was famous. In 1803, she began performing at Covent Garden, and was its leading star until her retirement in 1812 (her last role being Lady Macbeth). Tragedy was always her speciality. She continued to perform occasionally in retirement, and gave public readings.