Cecilia Margaretta Mostyn, née Thrale, was born on 8 February 1777, in Streatham, to Hester Thrale Piozzi (1741-1821) and Henry Thrale (1728-1781). After the death of her father in 1781, Cecilia was raised primarily by Hester, and Hester's second husband, Gabriel Piozzi (1740-1809), with whom she had a good relationship. After a childhood characterised by illness and stubbornness, Cecilia eloped at the age of eighteen and married (1795) John Meredith Mostyn (1775-1807) in Scotland, with issue; 3 sons, 1. died at birth, unnamed (1797), 2. Henry Meredith (Harry) Mostyn (1799-1840), 3. Thomas Arthur Bertie Mostyn (b.1801). The elopment caused a permanent rift with her parents, who only begrudgingly engaged with her new family. Cecilia's relationship with her sisters was less fraught, however, as evidenced by her collaboration with Hester Maria Thrale Elphinstone (1764-1857) and Susannah Arabella Thrale (1750-1858) in the foundation of the Thrale Almshouses, subsidised lodgings for elderly poor women in Streatham, in 1832. Cecilia died at Brighton Railway Station on 1 May 1857. She appears only tangentially in Montagu's correspondence with her mother, being only an infant before Montagu's own rift with Hester in the early 1780s.