 Appointed a soloist in 1963, Mason was promoted to principal dancer in 1968.The range of roles in her personal repertory was broad, encompassing the classicism of Odette/Odile in "Swan Lake" and Nikiya in "La Bayadère" as well as the austerity of purely abstract works such as "Song of the Earth", set by MacMillan to Gustav Mahler's meditative "Das Lied von der Erde".She was dramatically effective in such disparate roles as the ruthless Black Queen in "Checkmate" by Ninette de Valois, and the gentle Lady Elgar in "Enigma Variations (My Friends Pictured Within)", by Frederick Ashton.