The English contralto, Monica Sinclair, studied at the Royal Academy of Music. She made her operatic début in 1948, singing Suzuki in Madama Butterfly with the Carl Rosa Opera. The following year she joined Covent Garden, making her début as the Second Boy in The Magic Flute. During her first season she also sang Maddalena in Rigoletto, Mrs Sedley in Peter Grimes, Feodor in Boris Godunov, Suzuki and Rosette in Manon, Flosshilde in Das Rheingold and Siegrune in Die Walküre.
In 1950-1951, she added Azucena in Il trovatore and Pauline in The Queen of Spades, one of her best early roles, in which she accompanied herself on the harpsichord. She also took part in the first of several first performances, as a Heavenly Body in The Pilgrim's Progress (1951) by Ralph Vaughan Williams. In 1952 she sang Margret in the British stage premiere of Alban Berg's Wozzeck and appeared in two travesty roles, as Cherubino in The Marriage of Figaro and and as Herodias' Page in Salome.
Monica Sinclair sang the Countess of Essex in the first performance of Benjamin Britten's Gloriana in 1953; she sang Evadne in the world premiere of William Walton's Troilus and Cressida in 1954, and a Voice in the first performance of Michael Tippett's The Midsummer Marriage in 1955. Later she sang Sosostris in that opera as well. Other new roles during those years included Mercedes in Carmen and the Voice of Antonia's Mother in The Tales of Hoffmann. She also recorded several of the contralto roles in Gilbert and Sullivan. She made her début at Glyndebourne in 1954 as Ragonde in Rossini's Le Comte Ory, a comic role that suited her particularly well. Other parts that she sang at Glyndebourne included Berta in Il barbiere di Siviglia and Marcellina in Le nozze di Figaro (the same character in Beaumarchais' originals) in 1955, Dryade in Ariadne auf Naxos in 1957 and Queen Henrietta in Bellini's I Puritani, with Joan Sutherland as Elvira, in 1960.
Returning to Covent Garden in the 1959-1960 season, Monica Sinclair found several congenial new roles. The first was Annina in Der Rosenkavalier, in a revival conducted by Georg Solti, making his Covent Garden début, with Elisabeth Schwarzkopf and Sena Jurinac. Then came Bradamante in George Frideric Handel's Alcina, directed and designed by Franco Zeffirelli, with Joan Sutherland in the title role. Sinclair was an excellent Handel singer, as she had shown in 1959 as Juno in Semele for the Handel Opera Society, at Sadler's Wells Theatre. Further roles at Covent Garden included Theodosia, the housekeeper in Die Schweigsame Frau, the Old Prioress in Francis Poulenc's Dialogue des Carmélites, Marfa in Mussorgsky's Khovanshchina, Emilia in Otello and, best of all, the Marquise de Birkenfeld, with Joan Sutherland as Marie, and a youthful Luciano Pavarotti as Tonie. Sinclair accompanied Joan Sutherland on a tour of Australia in 1965. Arsace in Rossini's Semiramide at Florence in 1968 with Joan Sutherland in the title role was a mistake, one of the few in Sinclair's career.
Monica Sinclair was also successful in a wide variety of repertory on the concert platform. Thomas Beecham chose her for his last recording of Messiah, and for his sets of Mozart's Requiem and Delius's A Mass of Life. Her many recording credits include a fearsome Sorceress in the 1961 version of Dido And Aeneas, with Janet Baker as Dido. She also appeared on several occasions at the last night of the Proms, singing the solo in Rule Britannia.
Monica Sinclair had a formidable presence on stage, and seemed born to act tragedy and comedy with equal aplomb. What she may have lacked in vocal smoothness, she made up for with her command of character and technique. She married (and was divorced from) Anthony Tunstall, a former Covent Garden horn player. They had six children, all of whom survive her.