Catherine Louise Fink was born in St. Louis, Missouri in 1909, the second of the four children of Leo George Fink, an Austrian-born pawnbroker and jeweler, and his wife, the former Hattie A. Tetrick. Her siblings were Blanche, Marian, and Leo.
Thompson began her career in the 1930s as a singer and choral director for radio. Her first big break was as a regular singer on The Bing Crosby-Woodbury Show (CBS, 1933–34). This led to a regular spot on The Fred Waring-Ford Dealers Show (NBC, 1934–35) and then, with conductor Lennie Hayton, she co-founded The Lucky Strike Hit Parade (CBS, 1935) where she met (and later married) trombonist Jack Jenney.
In 1943, Thompson signed an exclusive contract with MGM to become the studio’s top vocal arranger, vocal coach, and choral director. She served as main vocal arranger for many of producer Arthur Freed’s MGM musicals and as vocal coach to such stars as Judy Garland, Lena Horne, Frank Sinatra, and June Allyson.
Thompson, who lived at the Plaza Hotel in New York City, became most notable as the author of the Eloise series of children’s books, which were partly inspired by the antics of her goddaughter Liza Minnelli, daughter of Judy Garland and film director Vincente Minnelli, but when asked if this was true responded, “I am Eloise”. The four books in the series, illustrated by Hilary Knight, are Eloise (Simon & Schuster, 1955), Eloise in Paris (Simon & Schuster, 1957), Eloise at Christmastime (Random House, 1958) and Eloise in Moscow (Simon & Schuster, 1959). They follow the adventures of the precocious six-year-old girl who lives at The Plaza. All were bestsellers upon release and have been adapted into television projects. She also composed and performed a Top 40 hit song, “Eloise” (Cadence Records, 1956). A fifth book, Eloise Takes a Bawth was posthumously published by Simon & Schuster in 2002, culled from Thompson’s original manuscripts once slated for 1964 publication by Harper & Row. However, at the time, Thompson was burned out on Eloise; she blocked publication and took all but the first book out of print, drastically reducing the income of her collaborator.
She returned to live in New York in 1969. Immediately following the death of Judy Garland, Kay appeared with her goddaughter Liza Minnelli in Otto Preminger’s Tell Me That You Love Me, Junie Moon (Paramount, 1970). In 1974, Thompson directed a groundbreaking fashion show at the Palace of Versailles featuring performances by Liza Minnelli and the collections of Halston, Bill Blass, Oscar de la Renta, and Anne Klein.