The daughter of a prosperous financier and consultant, McWilliams graduated from Smith College (B.A., 1934) and worked occasionally in advertising. During World War II, from 1941 to 1945, she performed clerical work in Ceylon (Sri Lanka) and China for the Office of Strategic Services (OSS, the forerunner of the Central Intelligence Agency), where she met Paul Cushing Child, whom she married in 1945. During the Childs' six-year postwar stay in Paris, she attended the Cordon Bleu cooking school for six months and studied privately with master chef Max Bugnard. She and two French friends, Simone Beck and Louisette Bertholle, in 1951 founded L'École des Trois Gourmandes (“The School of the Three Gourmands”) and later wrote the best-selling cookbook Mastering the Art of French Cooking, 2 vol. (1961, 1970), which was praised for its clarity and comprehensiveness. Her culinary crusade was stated plainly in her introduction:

The Childs settled in Cambridge, Massachusetts, in 1961, though they continued to visit Europe regularly and maintained a house in the south of France. From Boston's public television station she then initiated the immensely popular cookery series The French Chef, which premiered in 1963. It is credited with convincing the American public to try cooking French food at home. With her humour, exuberance, and unpretentiousness, Child became an unlikely star. Although she often made mistakes while cooking, she remained unflappable, encouraging viewers to accept mishaps and continue cooking. Child, who had a distinct warbling voice, ended each show with “Bon appétit!” Numerous television series followed, including Julia Child and Company, Dinner at Julia's, Baking with Julia, and In Julia's Kitchen with Master Chefs. She produced a book under the name of each of her shows and also wrote The Way to Cook (1989) and Cooking with Master Chefs (1993). Julia and Jacques Cooking at Home (1999) was cowritten with chef Jacques Pépin, with whom she also collaborated on television shows. Child was the recipient of numerous honours during her career, including a Peabody Award for her television work and a National Book Award. She also received the French Legion of Honor (2000) and the Presidential Medal of Freedom (2003).