Cathleen Mann was born in Newcastle upon Tyne on 31 December 1896 to the Scottish portrait painter Harrington Mann, the second of his three daughters. Her mother was the portraitist and interior director Florence Sabine Pasley. 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). Mann eventually became a member of the Royal Institute of Oil Painters and the Royal Society of Portrait Painters.
During the 1930s Mann also engaged in costume design for British films. Her work included The Iron Duke (1935) starring George Arliss and Things to Come (1937) starring Raymond Massey. Mann donated some of her costume design drawings to the Victoria & Albert Museum, where they are on display.