About forty portraits by Johnston are known to survive; many have preserved their original frames and backboards, on which her signature may be found. These mostly depict members of her social circle and, later, of her husband's Charleston congregation, such as Colonel William Rhett. Many of her South Carolina portraits depict members of Huguenot families that had settled in the New World, including the Prioleaus, Bacots (including Pierre Bacot and his first wife Marianne Fleur Du Gue) and duBoses (including Judith DuBose). Today, a number of her works are held by the Gibbes Museum of Art in Charleston, which has developed an interactive online exhibition dedicated to her work; other pieces may be seen in the Museum of Early Southern Decorative Arts, the Metropolitan Museum of Art, and the Greenville County Museum of Art. Johnston is not known to have worked in oils, but one of her portraits was copied at some point by Jeremiah Theus.