Adams was born on February 12, 1775, in the City of London, the illegitimate daughter of Joshua Johnson, an American merchant from Maryland, whose brother Thomas Johnson later served as Governor of Maryland and United States Supreme Court Justice, and Catherine Newth, an Englishwoman, whose identity was long a mystery; her great-grandson Henry Adams joked that her existence was "one of the deepest mysteries of metaphysical theology." She was baptized as Louisa-Catharine Johnson at the parish church of St Botolph without Aldgate on 9 March 1775, when her parents’ names were recorded as Joshua and Catharine and their address was given as Swan Street. She had six sisters: Ann "Nancy," Caroline (mother of Union General Robert C. Buchanan), Harriet, Catherine, Elizabeth (second wife of United States Senator John Pope of Kentucky), and Adelaide, and a brother, Thomas. She grew up in London and Nantes, France, where the family took refuge during the American Revolution.