At the age of 16 Mason joined the Royal Ballet as the Company's youngest member, and became a principal dancer in 1968. She worked closely with the choreographer Sir Kenneth MacMillan, who created the roles of Lescaut's Mistress in Manon, Calliope Rag in Elite Syncopations, Summer in The Four Seasons and the Midwife in Rituals for her.
Mason was also given the role of the Chosen Maiden in The Rite of Spring that had been originally created by Diaghilev for Lydia Sokolova in the 1920s. Her last leading role was as Nursey in Isadora by Kenneth MacMillan, in 1981, although she continued to appear in mime roles as a guest artist with the Royal Ballet for some time after that.
Following her retirement as a principal dancer, Mason continued to work for the Royal Ballet company and in 1984 she was appointed Principal Répétiteur to the company. In 1991 she became Assistant Director of the company under Sir Anthony Dowell. Following the company's move to its new headquarters at the Royal Opera House in London's Covent Garden, Mason replaced Dowell's successor, Ross Stretton, as Director in late 2002. She retired from the company in July 2012, having served the company for 54 years.