Born in New York City, Guarnaschelli learned to cook as a young girl while helping her mother Maria, a cookbook editor who worked on the seminal The Joy of Cooking among other high-profile projects, test recipes in the family kitchen. On the day she left school in 1991, Guarnaschelli went to work under the tutelage of American chef Larry Forgione, who encouraged her to travel the world in order to broaden her tastes. She went on to study at La Varenne Culinary School in France and work at the Michelin three-starred Guy Savoy restaurant in Paris. Returning to the United States to work as sous chef at Daniel in Manhattan, she worked her way up to the post of executive chef at both New York's Butter and The Darby restaurants, where she first gained the attention of Food Network producers and began her on-screen career. She was also an instructor and served on the Chef's Advisory Council at New York's Institute of Culinary Education.