Less well known than his brother, steel magnate Andrew Carnegie, Thomas Morrison Carnegie, Sr., married Lucy Ackerman Coleman, daughter of another industrialist, in 1866. Together they had nine children. By the time he died in 1886, Thomas had amassed a fortune and bought large amounts of land on Cumberland Island.

It is believed that the Carnegies read about the area in an article by Frederic Ober in 1880 and had perhaps visited Fernandina, Florida, one of the nearest ports, in the early 1880's. By spring 1881, Thomas M. Carnegie, Sr., and General W.G.M. Davis, owner of the old Dungeness property at the south end of the island, were corresponding about a purchase. The Carnegies subsequently built themselves a house there called Dungeness as a winter retreat.

The Carnegie Estate at one time resembled a small principality with the merchant prince's house being Lucy C. Carnegie's home, Dungeness, and each of her children's homes spread north along the island away from her. There was Stafford, rebuilt on the site of an earlier structure after a fire for William C. Carnegie and later belonging to Andrew Carnegie II; Greyfield for Margaret C. Ricketson and her children; Plum Orchard, built for George L. Carnegie and his bride and then passing to Nancy C. Johnston's family since George was childless; the Cottage for Thomas Morrison Carnegie, Jr.; and the Grange, eventually for Florence C. Perkins's children.

At various times the managers and their advisors tried to run the Estate as a business, a working farm, against which they could write off losses. When local governments tried to raise their property taxes, the Carnegies protested that their establishment brought business to the area and provided labor opportunities; however, they brought employees with them from the northeast, and for decades they continued to order food staples, machinery, and household goods from outside the South.

In actuality, Lucy C. Carnegie invested in a Pittsburgh office building so that the rent might finance her island retreat. Not only did she maintain that aim throughout her own lifetime (she died in 1916), but she also constructed her will in such a way that the Estate could continue to function as an entity until the death of her last child (1962).

Records creation, maintenance, and storage on the island from about 1885 until 1959 was the function of the manager or superintendent of the Estate. Both titles were used. Sometimes this man was in charge of many employees (including a separate superintendent for each house), all financial transactions, and even responsibilities connected with estates in other states and the Estate income from investments. William E. Page (born 1862), in the Carnegie employ from the late 1880's until he died in 1922, was such a manager. Beginning as a tutor for the sons, he won Lucy C. Carnegie's confidence and gradually handled more and more of her business. Page was educated at Haverford and Harvard. From 1913 on, the Estate had the help of an accountant to audit the books and handle increasingly complex income taxes. When Page died, the Carnegie children had neither the inclination nor the money to hire someone of this type again.