PERHAPS because she came to the profession relatively late and accepted only engagements with which she was closely in sympathy, the American mezzo-soprano Lorraine Hunt Lieberson was in her forties before she became known to British audiences.

Her breakthrough came when she played Irene in Handel’s Theodora; it was staged by the innovative and often controversial US director Peter Sellars at Glyndebourne in 1996, and she emerged as its star. She sang with such emotional truth and directness that many of the audience came away in tears.

A natural musician without ego or artifice, who sang to communicate, she embraced all sorts of music, but particularly championed Baroque operas and the work of living composers. She was meticulous in her preparation and it was often noted that she imbued the music she sang with new qualities which seemed to bring it more colourfully to life. The French vocal coach Denise Massé likened her to Callas in her determination “to dig as deeply as possible into the character — to find all the grain in the wood”.

She was born Lorraine Hunt in the San Francisco Bay area, the eldest of four children. Her mother was a singer and voice teacher, and her father a music teacher who conducted local amateur operas. She took up the piano, then the violin and finally the viola. Such was her father’s controlling influence that it took many years for her to find her own voice, she said, “But it was partly because of this upbringing that I came to care so deeply about genuine expression.”

After San José State University, where she majored in viola and voice, Hunt embarked on a career as a viola player. One of her first regular jobs was with her boyfriend. They played music by Chicago, Joni Mitchell, Stevie Wonder and Jobim in Los Gatos, with him on the guitar and her singing and playing the viola. She also formed a quartet with some friends, Novaj Kordoj (Esperanto for “new strings”) which played contemporary music.

She enjoyed recounting one episode in particular from this period: her boyfriend’s arrest for buying marijuana in Mexico. She joined other women in bribing the guards to let them live in the prison in shacks they put up in the grounds. (This experience, she said later, qualified her uncannily well to play Leonore in Fidelio.) She also recalled having a sort of spiritual awakening after reading Autobiography of a Yogi, by Paramahansa Yogananda: “The story of his spiritual journey set off sparks of recognition,” she said. “Things were lighting up.”

After a stint as the principal violist of the Berkeley Free Orchestra (during which time she volunteered to sing the part of Hänsel in a production of Humperdinck’s Hänsel und Gretel), she moved to Boston, where she was accepted into the opera programme at the Boston Conservatory.

She continued to work as a viola player: a church in the city, Emmanuel, had an impressive music programme — it put on a complete Bach cantata every Sunday in a cycle that ran over years — and she joined the orchestra. A regular collaborator with Emmanuel Music and its conductor Craig Smith was the director Peter Sellars. When he discovered that Hunt could sing, he cast her in his 1985 production of Handel’s Julius Caesar.

She played Sesto, a 14-year-old boy, full of rage and bent on avenging his father’s death. “I was all over the place,” she said later, “but it was a great opportunity to go to the edge, and actually go too far maybe, where most stage directors I know wouldn’t have gone, or would have been reining me in.”

She demonstrated the rich quality of her voice and her ability to sound a note from nothing — which the Emmanuel choir director Craig Smith attributed to her years of playing the viola. She was experiencing completely, it seemed, the emotions of her character, and often came off stage in tears. Everyone who saw it wanted to know who she was.

The production was transforming for both Sellars and Hunt, and when Hunt’s viola was stolen in 1988 she took it as a sign that she should take up singing full-time.

She was soon in great demand. She worked again with Sellars, as Donna Elvira in Don Giovanni in 1987. She also worked with Mark Morris’s dance group, in radical stagings of Purcell’s Dido and Aeneas and Handel’s L’Allegro, il Penseroso ed il Moderato. She sang Charpentier’s Médée with Les Arts Florissants, conducted by William Christie in 1993, and Carmen at the Boston Lyric Opera in 1994. She also appeared in Peter Lieberson’s first opera, Ashoka’s Dream, for the Santa Fe Opera in 1997, and two years later married the composer.

In 1999 she played Myrtle Wilson, the mistress of Tom Buchanan, in John Harbison’s The Great Gatsby at the Met, and won glowing reviews for her portrayal of Lucretia in The Rape of Lucretia at the Edinburgh festival the same year. She also made a series of recordings of operas and oratorios by Handel with Nicholas McGehan and the Philharmonia Baroque Orchestra.

In 2001 she made one of her infrequent visits to London, where she sang two of J. S. Bach’s solo cantatas at the Barbican in stagings directed by Sellars. He had turned them into performance pieces: while singing Mein Herz schwimmt in Blut (BWV199) she danced a mime reminiscent of those in Chinese opera. For Ich habe genug (BWB82) she lay on her back in a white hospital gown, the tubes of the life-support machine that has just been turned off hanging beside her.

The performance, according to Sellars, was about the last half-hour of a human life “where the dissolution of the body brings as ultimate release from pain and a step forward not back”. It was, inevitably, controversial — but it was lent poignancy by the death of Hunt Lieberson’s sister from breast cancer the year before.

Hunt Lieberson herself had also been found to have the disease, but after treatment she returned to performing; later roles included the première of Peter Lieberson’s Neruda Songs and the Berlioz Romeo and Juliet with the Boston Symphony Orchestra under James Levine. She was Ottavia in L’Incoronazione di Poppea at the Aix-en-Provence Festival and at the San Francisco Opera; and took part in the world premiere of El Niño by John Adams at the Châtelet in Paris and in San Francisco. She appeared in Elgar’s The Music Makers at the first night of the Proms in 2004 (after which one critic called her “the ultimate Gerontius angel”).

Being ill, she found, had changed the way she sang. Until then she had been dogged by perfectionism — “Silly, really, because perfection isn’t what moves me when I listen to inspirational singers like Janet Baker, Christa Ludwig or Joni Mitchell” — but now she tried to let go. She also reduced the number of engagements she accepted, and moved with her husband from Halifax, Nova Scotia, to Santa Fe, in the hills of New Mexico.

She gave her last performances on tour with the Boston Symphony Orchestra in March, singing music composed by her husband. Soon after her return she was found to have metastatic liver cancer.

She is survived by her husband.

Lorraine Hunt Lieberson, mezzo-soprano, was born on March 1, 1954. She died of cancer on July 3, 2006, aged 52.