She was the singing voice of leading actresses on the soundtracks of several musicals, including Deborah Kerr in The King and I and An Affair to Remember, Natalie Wood in West Side Story, and Audrey Hepburn in My Fair Lady, although her roles were concealed from audiences when the films were released.

[1] In 1947, having adopted the stage name "Marni Nixon", she made her Hollywood Bowl solo debut in Carmina Burana with the Los Angeles Philharmonic under conductor Leopold Stokowski.

[4] Nixon's career in film started in 1948 when she sang the voices of the angels heard by Ingrid Bergman in Joan of Arc (1948).

The same year, she did her first dubbing work when she provided Margaret O'Brien's singing voice in 1948's Big City and then 1949's The Secret Garden.

She sang for Jeanne Crain in Cheaper by the Dozen (1950) and dubbed Marilyn Monroe's high notes in "Diamonds Are a Girl's Best Friend" in Gentlemen Prefer Blondes (1953).

[11] After a court case, she received royalties from sales of the soundtrack album and spoke out for the rights of ghost singers.

[15] Nixon made guest appearances with Leonard Bernstein's Young People's Concerts, including in 1960, singing "Improvisation sur Mallarmé I" from Pli selon pli by Pierre Boulez,[3] and on April 9, 1961, in a program entitled "Folk Music in the Concert Hall", singing three "Songs of the Auvergne" by Joseph Canteloube.

[16] Before My Fair Lady was released in theatres in 1964, Nixon played Eliza in a revival of the musical at New York City Center.

In the DVD commentary to the film, director Robert Wise comments that audiences were finally able to see the woman whose voice they knew so well.

[1][3] On stage, in 1984, she originated the role of Edna Off-Broadway in Taking My Turn, composed by Gary William Friedman, receiving a nomination for a Drama Desk Award.

[23] In regional theatre and Off-Broadway, she played Nurse in Romeo and Juliet and appeared in productions of The King and I and The Sound of Music.

[11] She performed in the 2008 North American Tour of Cameron Mackintosh's UK revival of My Fair Lady in the role of Mrs.