She then studied in New York City, with Scottish artist J.B. Wandesforde, who taughter her how to paint miniatures. Cornelia Fassett spent three years in both Paris and Rome studying under Giuseppe Castiglione, Henri Fantin La Tour, and Lambert Joseph Matthew. After an early career in Chicago, Fassett and her husband moved to Washington, D.C., in 1875, where she painted successful documentary portraits of notable government figures and he was photographer to the Supervising Architect of the Treasury. A year after their move, her 1876 group portrait of the Supreme Court justices was exhibited at the Philadelphia Centennial Exposition.