She was the daughter of Thomas Davies, a carver and gilder in Birmingham, and was born there on 16 December 1762. Her father died in a madhouse while she was a small child. Her mother kept a tavern frequented by actors, and among others by Richard Yates, under whose management Mary appeared at the Birmingham Theatre as the young Duke of York in Richard III, playing subsequently Cupid in William Whitehead's Trip to Scotland, and Arthur in King John. After visiting Bath and York she went to Gloucester, where she played Juliet to the Romeo of an actor named Wells, to whom she was married in St. Chad's Church, Shrewsbury. Wells shortly afterwards deserted her.
On 1 June 1781, as Madge in Isaac Bickerstaffe's Love in a Village and Mrs. Cadwallader in Samuel Foote's Author, she made her first appearance at the Haymarket. John Genest says that she was excellent in both characters. Jenny in Lionel and Clarissa (Bickerstaffe) followed, and on 3 September in John O'Keeffe's Agreeable Surprise she was the first Cowslip, a name that stuck to her (though she is occasionally spoken of as ‘Becky’ Wells). Genest wrote that nothing could be superior to her acting as Cowslip and that of John Edwin as Lingo.
On 25 September, as Nancy in Love in a Camp, she made her first appearance at Drury Lane, where also she played on 29 October. Jenny in the Gentle Shepherd, adapted from Allan Ramsay by Richard Tickell. Harriet in the Jealous Wife, Widow O'Grady in the Irish Widow, Flora in She Would and She Would Not (Colley Cibber), and Jacintha in the Suspicious Husband followed. At the Haymarket in 1782 her name appears to Molly in the English Merchant and Bridget in the Chapter of Accidents (Sophia Lee). She also, as she says, replaced Mrs. Cargill, who had eloped, as Macheath in the Beggar's Opera, with the male characters played by women and vice versa. She made a distinguished success, and was received with great enthusiasm. She played at Drury Lane Kitty Pry in The Lying Valet, and Jane Shore on 30 April 1783, her first appearance in tragedy. At the Haymarket she was on 6 July 1784 the original Fanny in Elizabeth Inchbald's Mogul's Tale, on 6 September the first Maud in O'Keeffe's Peeping Tom, the eponymous Isabella, and Lady Randolph in Douglas.
Nancy Buttercup, an original part in O'Keeffe's Beggar on Horseback, was seen at the Haymarket on 16 June 1785. On 14 December she made her first appearance at Covent Garden as Jane Shore (in her own opinion, her best performance), playing also Laura in Edward Topham's farce The Fool, which her acting commended to the public. After repeating Lady Randolph and Isabella, she was on 5 January 1786 Imogen in Cymbeline; William Woodfall in the Morning Chronicle awarded her praise for the performance. Andromache in the Distressed Mother (Ambrose Philips) followed, and was succeeded by Shakespearean heroines (Rosalind, Portia), and Fidelia in the Plain Dealer; and she was on 24 April the first Eugenia in The Bird in a Cage, or Money works Wonders, altered from James Shirley. At the Haymarket in 1786 she played some unimportant original parts. When John Palmer made in 1787 his trial effort at the Royalty Theatre, Wellclose Square, she gave her imitations of Mrs. Siddons and other actresses, and was paid £50 a night.
She came back to Covent Garden, where she was on 17 September 1787 Mrs. Page in the Merry Wives of Windsor and played Lady Percy, Lady in Comus, Rosina, Anne Lovely, and Fatima in Cymon. Here she remained some time, acting in the summer at Cheltenham, Brighton, and Weymouth, where she was favoured by royalty.
Her domestic affairs became complicated. She had entered into close relations with Edward Topham, a captain in the Guards, who was concerned in a daily newspaper called the World, in the production of which she assisted. She had also backed bills for a considerable amount for her brother-in-law, the husband of a Miss Davies (who appeared at the Haymarket on 28 July 1786 as Amelia in the English Merchant). This guarantee involved her in endless trouble. More than once she was a prisoner in the Fleet Prison and in other places.
In the Fleet she met Joseph Sumbel, her second husband, who was confined there for contempt of court. Sumbel was a Sephardic Jew, secretary to the ambassador from Morocco, and the wedding was performed in the Fleet. A year later he sought unsuccessfully to have the marriage annulled or dissolved, declaring that she was not his wife; she meanwhile had embraced Judaism and taken the name of Leah. She subsequently became a Catholic or Methodist.
She does not seem to have acted much later than 1790, though she gave her imitations at private houses; and once attempted to give them publicly during Lent, but was prevented by the bishop of London.
She spent her later years in lodgings with her aged mother. She also applied to the Covent Garden Theatrical Fund, and received an annuity of £55 until her death in London on 23 January 1829. She was buried in St Pancras, London.
She published in 1811 ‘Memoirs of the Life of Mrs. Sumbel, late Wells, of the Theatres Royal Drury Lane, Covent Garden, and Haymarket, written by herself,’ (London, 3 vols.) The three volumes of this rambling autobiography are occupied principally with details of travels in search of her children, who refused to know her, or of friends. The remainder stock seems to have received a new title-page in 1828, when it appeared as ‘Anecdotes and Correspondence of Celebrated Actors and Actresses, including Mr. Reynolds, Mr. Kelly, Mr. Kemble, Mr. Colman, Mrs. Siddons, &c. Also an Account of the Awful Death of Lord Lyttelton.’
Her portrait, in the character of Cowslip in the ‘Agreeable Surprise,’ was engraved by John Downman (Bromley, p. 447). A portrait of her by Dewilde, as Anne Lovely in ‘A Bold Stroke for a Wife,’ is in the Mathews collection in the Garrick Club. An engraving by J. R. Smith from his own picture of her as Cowslip was published by Ackerman in 1802.