Mary Davies was born in Birmingham, where her father Thomas Davies was an artisan and her mother ran a tavern frequented by actors. Mary obtained a job as an actress with the Birmingham Theatre. Her sister Anna Davies also became an actress. Mary's first role was as the child Richard, Duke of York, in Shakespeare's Richard III. On tour of the provinces, Mary played Juliet to the Romeo of Ezra Wells, whom she married at about age 16 in St. Chad's Church, Shrewsbury. Wells deserted her shortly afterwards. Mary made her way to London and gave her first performances on the London stage in 1781 as Madge in Isaac Bickerstaffe's play Love in a Village and as Mrs. Cadwallader in Samuel Foote's Author at the Haymarket. She played Jane Shore at Covent Garden (her best performance, in her opinion). She was the original Cowslip in O’Keeffe and Arnold’s The Agreeable Surprise, and the nickname "Cowslip" stuck with for her many years. She was popular with the public in a variety of musical, comic and tragic roles. However, her personal life was less successful. She lived for a few years with Edward Topham, an occasional playwright, with whom she had
four children. In 1796, she wound up in the Fleet Prison for debt. There she met Joseph Sumbel, a Sephardic Jew who was the former secretary to the Moroccan ambassador. Mary converted to Judaism and they were married in the prison in 1798. A year later, Sumbel tried to have the marriage annulled. He died in 1804, and Mary returned to Christianity. She published her 3-volume autobiography in 1811 as "Memoirs of the Life of Mrs. Sumbel, late Wells, of the Theatres Royal Drury Lane, Covent Garden, and Haymarket, Written by Herself." It later appeared as "Anecdotes and Correspondence of Celebrated Actors and Actresses, including Mr. Reynolds, Mr. Kelly, Mr. Kemble, Mr. Colman, Mrs. Siddons, &etc. Also an Account of the Awful Death of Lord Lyttelton" (1828).