"Slim" Keith, née Nancy Gross, was born in Salinas, California, the daughter of a cannery owner. She was called the original "California Girl" for her golden-haired, long-limbed good looks and athletic ability. She left school at age 16 and was vacationing alone in Death Valley when she met actors William Powell and George Brent, and William Randolph Hearst, who took her to Hollywood parties. During her life, she was married to three important men: director Howard Hawks, with whom she had a daughter; theatrical agent-producer Leland Hayward; and British banker Sir Kenneth Keith, who made her Lady Keith. She became a popular socialite and style icon with many actor and artist friends, included Gary Cooper, Gregory Peck, Cary Grant, Clark Gable, Rosalind Russell, Lauren Bacall, Babe Paley, Truman Capote, Irving "Swifty" Lazar, Charles Lindbergh, Robert Capa, Man Ray, and Ernest Hemingway. In 1946, she was named the best-dressed woman of the year by America's fashion editors. She was credited with discovering and promoting Lauren Bacall, Joanne Dru, and Ella Raines, all three of whom resembled her somewhat.
Her memoir, entitled Slim: Memories of a Rich and Imperfect Life, written with Annette Tapert, was published shortly before her death in 1990.