After study in Italy, she began to sing opera in Europe. Returning to England, Masterson performed as principal soprano with the D'Oyly Carte Opera Company from 1964 to 1969, becoming popular with audiences and participating in several of the company's recordings, as well as those of Gilbert and Sullivan for All and the BBC.
She next joined English National Opera and went on to an international opera career lasting more than three decades. Although she performed a wide variety of roles, she was best known for her roles in the French repertoire and the works of Handel, as well as Gilbert and Sullivan. Her recordings include, in addition to many opera roles, operettas and musical theatre. She has been awarded a CBE and is a Fellow of the Royal College of Music.