A British portrait painter and costume designer for film. She was a member of the Royal Institute of Oil Painters and the Royal Society of Portrait Painters.
Her father Harrington Mann gave Cathleen painting lessons in his London studio, as did the portrait painter Ethel Walker. Walker continued to tutor Mann even while Cathleen was studying at Slade School of Fine Art in London. Walker remained an influence on Mann and the two often exhibited in the same group exhibitions. Mann's art career was put on hold owing to the First World War, when she worked with an ambulance unit.
 
By 1924 Mann had two portraits in the Royal Academy, and exhibited there regularly from 1930. Her work was displayed at the Victoria and Albert Museum, the Musée du Luxembourg, and the Royal Glasgow Institute of the Fine Arts. Two of her portraits hang in the National Portrait Gallery: Sir Matthew Smith and Sir Eduardo Paolozzi (both oil on canvas, 1952).