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.