Breakfast cereal he launched the Cornflakes brand, which overnight captured a national market. Soon there were forty rival manufacturers in the Battle Creek area. His brother William K. Kellogg (1860–1951) worked for him for many years until, in 1906, he broke away, bought the rights to Cornflakes, and set up the Kellogg Toasted Corn Flake Company. William Kellogg discarded the health food concept, opting for heavy advertising and commercial taste appeal. Later, his signature on every package became the company trademark. The second major innovator in the cereal industry was Charles W. Post, a salesman who was admitted to Kellogg's sanitarium as
Fay Kellogg death in 1901, Kellogg set up business for herself, establishing her own office in 1903. One of her first commissions was the renovation and construction of seven buildings on Park Place in Manhattan for the American News Company. She was soon in charge of all their building and renovation work in the United States. For jobs within 200 miles of New York City, Kellogg would supervise directly. For projects further afield, she would draw plans. Kellogg also helped design the Woman's Memorial Hospital in Brooklyn, founded by female physicians in 1881, as well as hundreds of other buildings and cottages.