Caselotti was born in Bridgeport, Connecticut to an Italian American family. Her father, Guido Caselotti (1884-1978), was an immigrant from Udine, and worked as a teacher of music and a vocal coach. Her mother, Maria Orefice (1893-1961), from Naples, was a singer in the Royal Opera Theatre of Rome. Her older sister, Louise, sang opera and gave voice lessons (for example, to Maria Callas, from 1946 to 1947). When Caselotti was seven years old, her family left Connecticut for Italy while her mother toured with an opera company. Caselotti was educated at an Italian convent, San Getulio, near Rome, while her mother performed in the opera. When her family returned to New York three years later, Caselotti relearned English and studied singing with her father. In 1935, after a brief stint as a chorus girl at MGM, Walt Disney hired Caselotti as the voice of his heroine Snow White. She was paid a total of $970 for working on the film (now worth approximately $15,967). She was under contract with Disney, and Disney prevented her from appearing in further film and other media, even for Disney, after Snow White and the Seven Dwarfs. Jack Benny specifically mentioned that he had asked Disney for permission to use her on his radio show and was told, “I’m sorry, but that voice can’t be used anywhere. I don’t want to spoil the illusion of Snow White.” The only other work Caselotti did following her premiere was an uncredited role in MGM’s The Wizard of Oz (1939); she provided the voice of Juliet during the Tin Man’s song, “If I Only Had a Heart”, speaking the line, “Wherefore art thou Romeo?” In 1946, she had an uncredited role in Frank Capra’s It’s a Wonderful Life, singing in Martini’s bar as James Stewart was praying.

Caselotti did several promotional spots for Snow White and the Seven Dwarfs, signing memorabilia. On November 22, 1972 (Thanksgiving Day), she guest-starred on an episode of The Julie Andrews Hour saluting the music of Walt Disney, singing “I’m Wishing” and “Someday My Prince Will Come” with Julie Andrews. She also made a guest appearance on the syndicated The Mike Douglas Show. Caselotti later wrote a how-to book, Do You Like to Sing?. Later in life, she sold autographs and also made an attempt at an opera career (for example, Rigoletto). In the early 1990s, when the Snow White Grotto at Disneyland was refurbished, Caselotti re-recorded “I’m Wishing” for the Snow White Wishing Well at the age of 75. In 1994, she was named a Disney Legend. Caselotti was married four times. Her first husband was Robert Chard, a local theater ticket broker whom she married in 1945. The marriage ended in divorce. She later met bit actor Norval Mitchell, whom she married in 1952. He retired after marrying her, and died in 1972. The same year, she was married to a podiatrist, Dr. Joseph Dana Costigan, who later died in 1982. Caselotti married her last husband, Florian St. Pierre, a retired postal employee, in 1989 and later divorced. On January 18, 1997, Caselotti died of respiratory failure from lung cancer at her Los Angeles home.