Darby was a singer, arranger, songwriter, composer, choral director, and conductor. Born in Nebraska, he was educated at Christian College. In 1929 he formed the quartet the King's Men, who performed in concerts and films and on radio and recordings. Later Darby formed the Ken Darby Singers, who pursued a similarly active musical course. He used both groups for the sound of the Munchkins in the Oz film.
Darby took a job with MGM in 1937; The Wizard of Oz was his first major project. He later worked for Walt Disney Pictures, on Song of the South (1946) and other projects — though Disney himself criticized Darby's proposed score for Johnny Appleseed (1948). Darby was nominated for Academy Awards six times, and won three Oscars for his music, for the films The King and I (1956) and Camelot (1967), awards he shared with Alfred Newman, and Porgy and Bess (1959), shared with Andre Previn. He also won a Grammy Award for the recording from the latter film, again shared with Previn.
He worked on many well-known movies of his era, including Carousel (1956), South Pacific (1958), How the West was Won (1962), and The Greatest Story Ever Told (1965). He also did extensive work for television in the 1950s and '60s.
As a popular songwriter, Darby's best known piece would likely be the Elvis Presley hit Love Me Tender. Darby wrote the lyrics to the song's traditional tune, though he credited them to his wife Vera Matson.