Marni Nixon, the American cinema's most unsung singer, died on Sunday in Manhattan. She was 86.

The cause was breast cancer, said Randy Banner, a student and friend. Ms. Nixon, a California native, had lived in Manhattan, on the Upper West Side, for more than 40 years.

Classically trained, Ms. Nixon was throughout the 1950s and '60s the unseen - and usually uncredited - singing voice of the stars in a spate of celebrated Hollywood films. She dubbed Deborah Kerr in "The King and I," Natalie Wood in "West Side Story" and Audrey Hepburn in "My Fair Lady," among many others.

Her other covert outings included singing for Jeanne Crain in "Cheaper by the Dozen," Janet Leigh in "Pepe" and Ida Lupino in "Jennifer." "The ghostess with the mostest," the newspapers called her, a description that eventually began to rankle.

Before her Hollywood days and long afterward, Ms. Nixon was an acclaimed concert singer, a specialist in contemporary music who appeared as a soloist with the New York Philharmonic; a recitalist at Carnegie, Alice Tully and Town Halls in New York; and a featured singer on one of Leonard Bernstein's televised young people's concerts.

Her concerts and her many recordings - including works by Schoenberg, Stravinsky, Webern, Ives, Copland, Gershwin and Kern - drew wide critical praise. Yet as late as 1990, decades after Ms. Nixon had made good on her vow to perform only as herself, she remained, in the words of The Los Angeles Times, "the best known of the ghost singers."

At midcentury, Hollywood was more inclined to cast bankable stars than trained singers in films that called for singing. As a result, generations of Americans have grown accustomed to Ms. Nixon's voice, if not her face, in standards like "Getting to Know You," from "The King and I"; "I Feel Pretty," from "West Side Story"; and "I Could Have Danced All Night," from "My Fair Lady."

Deborah Kerr was nominated for an Academy Award in 1956 for her role as Anna in "The King and I"; the film's soundtrack album sold hundreds of thousands of copies. For singing Anna's part on that album, Ms. Nixon recalled, she received a total of $420.

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"You always had to sign a contract that nothing would be revealed," Ms. Nixon told the ABC News program "Nightline" in 2007. "Twentieth Century Fox, when I did 'The King and I,' threatened me." She continued, "They said, if anybody ever knows that you did any part of the dubbing for Deborah Kerr, we'll see to it that you don't work in town again."

Though Ms. Nixon honored the bargain, her work soon became one of Hollywood's worst-kept secrets. She became something of a cult figure, appearing as a guest on "To Tell the Truth" and as an answer to clues featured by "Jeopardy!," Trivial Pursuit and at least one New York Times crossword puzzle.

Her increasing renown helped bring her spectral trade into the light and encouraged her to push for official recognition. "The anonymity didn't bother me until I sang Natalie Wood's songs in 'West Side Story,' " Ms. Nixon told The Times in 1967. "Then I saw how important my singing was to the picture. I was giving my talent, and somebody else was taking the credit."

Although the studios seldom accorded Ms. Nixon the screen credit and royalties that she began to demand, both became customary for ghost singers.

Marni Nixon and Dick Latessa in the Encores! production of the musical "Music in the Air" at City Center in 2009.  Sara Krulwich/The New York Times