She remained with the company for a decade, beginning with small roles such as Mercedes in Carmen and moving up to leading parts including Carmen, Octavian (Der Rosenkavalier), Marina (Boris Godunov), Magdalene (Die Meistersinger), and Mrs Sedley (Peter Grimes).
[1] Away from Covent Garden, Shacklock appeared in opera in Berlin with Erich Kleiber, and sang in oratorio, notably as the angel in The Dream of Gerontius with Sir John Barbirolli.
In 1961, Shacklock left the operatic stage and joined the cast of The Sound of Music for an unexpected six-year run as the Mother Abbess at the Palace Theatre, London.
While teaching at the Royal Academy of Music she became close friends with a young mezzo-soprano from Birmingham called Jean Tredaway, whom she subsequently adopted.
A vast archive of personal documents once belonging to her was discovered in a Midlands property and was auctioned on 18 August 2010 in Lichfield.